How would these somewhat rare boys' names go over in the UK?

In my humble opinion...

Cedric - I think there's a dragon in a story book and a character in harry potter called this.
Chester - name of a city in the uk
Ivan - like!
Kieran - very very common here!
Laszlo - people might not know how to say this
Malcolm - old fashioned but not yet retro cool
Marshall - surname
Maurice - like!
Munro - surname
Rye - I personally dislike this - just my taste
Trevor - like!

I pity any poor Trevors reading this. Come to think of it, the last Trevor I knew was black.

and YY to Trevors being quite racest despite having a black mate down the pub who is "allright".. and maybe a Thai wife..

I love your well-crafted portrait of a typical Trevor, freerange.

I agree that children don't hear what we hear in names.. they don't have the same associations
but teachers do.. and some teachers do pigeon-hole kids.. just sayin!

Cedric sounds like the sort of school swot who 'told' on everyone else. It's not a 'nice boy' name. Sorry.

Cedric - don't!
Chester - OK, know someone called this because his parents believed in numerology, sounds a bit American.
Ivan - OK but I would assume E European origin
Kieran - Fine, quite common
Laszlo - personally I like it but very E European to English ears
Malcolm - known a couple of very pleasant ones, but out of fashion
Marshall - OK, sounds American
Maurice - don't!
Munro - quite like this, Scottish surname, never come across it as a name
Rye - like the bread? Perhaps not!
Trevor - was at primary school with a nice Trevor. Bit boring but might be due a comeback.

Just to say that just because adults associate names with a particular stereotype doesn't mean to say that children will.

One of my DCs has a Malcolm in his class; the other has a Trevor (yes, really.) Now, to our adult ears both those names sound terribly old-fashioned and are normally only used for men who are 50+. To my mind, Malcolm is a bit of a "wet" name and Trevor is a bit rough. But funnily enough I've never heard any child make a comment about either name. I suppose when you are little you are constantly encountering new names and Malcolm and Trevor sound no stranger to them than Gabriel, Finlay or Noah (of which there are loads.)

From your list my favourites are Cedric and Ivan. I like Laszlo, but your DS would have to constantly spell it for people who haven't travelled to Eastern Europe.

Cedric is really awful as far as UK goes-I think of it as a bit of a joke along with Cecil , Claude and Cuthbert.
For me the best two on your list are Chester and Ivan.
Kieran is OK and so is Marshall.
Trevor is a bit dated now.
The others don't ring any alarm bells.

Cedric: wet and weedy. Posh. Old.
Chester: I know one teenager named this. He's a great kid but his name is not cool. It also rhymes with molester.
Ivan: I like this.
Kieran (not really rare, I know): I don't like this spelling of Ciaran, which is a lovely Irish name and I don't think it would cause many difficulties to use the original spelling in the UK. Kieran is a bit common.
Laszlo: weird but cool
Malcolm: My mum's cat's name. She chose it because it was funny. This name would be fine of a fifty-five year old man (and I say that as a Scot)
Marshall: American
Maurice: see Malcom
Munro: a hill. I've seena lot of Scots with first names that are surnames, but never come across a Munro.
Rye: bread
Trevor: see Malcolm/Maurice but multiply by a hundred million.

I feel I need to say more about Trevor. Trevor is sixty years old and has a potting shed. He eats his elevenses at eleven o'clock. He watches WW2 documentaries on the Discovery channel, and he and his wife Joyce are active members of their local church's social committee (CofE, of course); however, they are so bossy and domineering that fellow members Eileen and Malcolm feel it's time to have a word with the vicar. When not in church, eating elevenses or watching WW2 documentaries, Trevor is penning polite but somewhat racist letters to his local MP, mainly on the subject of immigration.

You are so right about Trevor, cherie.

Cedric - old and wet
Chester - to place namey / surnamey
Ivan - okayish
Kieran (not really rare, I know) - went to uni in the UK with one, didnt' cause anyone any pronounciation probs
Laszlo - sounds like a cafe Laszlo's... yip its more of something you'ld call a business
Malcolm - bit nerdy
Marshall - okay
Maurice - usually shortened to Moss, again bit wet
Munro - just.. no
Rye - girl or place
Trevor - wheeler dealer, slightly dodgy character, makes me think of a man in a pub saying "my mate trev can sort you out for one of those on the cheap"

I know a Kieran. He is my brother, goes to college, works and isn't 'chavvy' at all. He is 17.

I know an Ivan. He is so far up his own arse also my uncle who is in his 40's.

Names don't mean anything, been named something doesn't mean you get that personality. I don't like the rest though.

Cedric is cool, makes me think of cedric the entertainer

I think you have some great names here, but just please don't call your child Trevor.. It will get shortened to Trev for a start, and the associations of Kev, Baz, Wayne type 70s 80s names is just awful..

For all of you saying Chester sounds American, maybe 70-120 years ago, but not in this century. Maurice isn't of this century, either. I don't know a single person with either of those names, and I know some really old people. I know a Mauricio, but he's Latin American.

Marshall and Trevor, OTOH, I'm perfectly willing to concede as quite American.

3 of these names belong to my 60+ age Uncles OP if that was the type of thing you were after Also, another is my exP's (and dd's) surname.

Of them all I prefer Kieran.

I love Laszlo, it was on my list, but my DH got his veto out. Though mainly because it sounded odd with DTB's twin sister's name, and because we have no Hungarian heritage.
Good luck with your choices!
I think Malcolm and Trevor have the most boring cultural associations in the UK. The rest are cool - although Marshall just makes me think of Air Marshalls.

Cedric - well it may be coming back but it would be pretty ahead of the fashion in most parts of the UK.
Chester - American. Not a bad thing.
Ivan - OK. My association would be Russian but probably that's only from American submarine thrillers.
Kieran - naughty boy. This is a personal one, not everyone will feel the same. IMO it is not a glamorous name.
Laszlo - unusual but I think it's a good choice, although it depends how much it bothers you that it will be misspelt. A lot.
Malcolm - Again personally I think this is even more too far ahead of the curve - Malcolm was popular in perhaps the 40s and I don't think those names are coming back just yet.
Marshall - American and I really like it. Great choice.
Maurice - For me this is either E M Forster and not in a good way (I loathe EM Forster though so again it's personal), or Michael Caine's original name which he changed because it sounded dull I think. Again to me it is a 40s/50s name and rather timid-sounding.
Munro - Scottish ancestry assumed. Like it.
Rye - Kentish ancestry assumed. Would rather wonder, why Rye? Not attractive-sounding and unusual without being interesting.
Trevor - I actually associate with Trebor Mints, and also with a character in a TV play who was named Trevor because this indicated how ordinary, dull and suburban he was. Again a 50s name - too soon for this one IMO.

I'm not from the UK either but am a native English speaker so here's my tuppence worth:

Cedric
Chester
Laszlo
Marshall
Maurice
Rye
Trevor

I don't personally like these, some of them sound very old-fashioned and not in a cool way (Trevor, Cedric, Maurice) and some sound very American (Chester, Marshall). Laszlo sounds like a Hungarian goldsmith or something

Ivan
Kieran
Malcolm
Munro

I like all of these and they have a good solid sound to them. The last three are Irish, Scottish and a Scottish surname that would work well as a first name. Malcolm is the one I like best, it's a good name, traditional and with cultural background but not too trendy.

Cedric - can come across as a bit wet; also a Harry Potter character
Chester - place name
Ivan - like (but check it doesn't become risible with surname if also a noun)
Kieran - ok
Laszlo - ok (possibly a bi Jilly Cooper)
Malcolm - old fashioned in a fuddy duddy way
Marshall - seems rather American
Maurice - ok-ish, but old fashioned
Munro - ok
Rye - a bread
Trevor - very out of fashion, would seem odd

Clevor Trevor: from the late great Ian Dury.

Ivan, Kieran and Laszlo are all fine. Your other names not really.

Trevor, no no you just can't! Poor child. People WILL laugh, trust me. It gives the image of a small town business man aged at least 65 and is a bit of a 'joke' name in the UK.
Maybe one day it just might possibly be seen differently, but that time is MANY years from now.

Cedric - Little Lord Fauntleroy
Chester, Marshall and Munro are just dreadful
Ivan - OK
Kieran - OK but I prefer Ciaran
Laszlo - OK, especially if you have Hungarian connections
Malcolm, Maurice and Trevor are terrible, late-middle-aged names.

Rye is the worst of the lot. It doesn't even sound like a surname-first name, it just sounds like you might be aliens who picked something that could have been a name (a la Ford Prefect).