I used to work in the industry so will advise based on that and my own wedding - but both over a decade ago so some info may no longer apply.
Depending where you are usually - get married in Scotland. You can literally marry anywhere!
There are some stunning venues including beaches, small islands, castles (not necessarily expensive)
www.visitscotland.com/accommodation/wedding-venues/castles/
"Gretna green :)" although not this - not cheap, booked WAY in advance, over commercialised now.
Or consider venues like church or village halls - which are often cheap to hire, can be very plain decor but it's very easy & cheap to dress them up - balloons, paper decorations, coloured tablecloths etc. They also don't insist you use their caterers, their dj, their fancy chairs (with extra hire costs!) etc
This is what we did, the planning time coincided with a big calendar event which meant as soon as the event was over I was able to snap up loads of decorations for pennies!
Don't assume a dress from a boutique will be expensive - actually if you shop between now & Easter there can be real bargains to be had because they're getting rid of stock (including samples which are heavily reduced mine was an ex sample dress and I paid £300 for a £3000 dress) to make room for the new season BUT do NOT skimp on the seamstress! If you're having a traditional gown the styles and fabrics are often not ones that regular high street seamstresses are used to handling. I've had to calm brides who took £10k silk gowns to the woman on the high street who mainly does hems on school uniforms and zip replacements - and then they wonder why their dress is ruined!
"Don't buy your dress from China to save money, it's a false economy as even if it's not rubbish it will probably need to be altered" if cautious and follow recommendations from trusted people this can be OK - it's where many boutiques get their gowns from! And almost all dresses need alteration anyway especially as they're usually made based on a bridal height of 5' 10" as you can shorten a full length gown but you can't add length. Brides taller than 5' 10" usually need to order bespoke.
Equally you could think outside the box and get a maxi dress, go fifties/sixties shorter style, as I mentioned on a recent thread I've seen all sorts inc brides in tuxes and various costumes.
Shoes & accessories - frankly online is best but get recommendations and read reviews closely. Certainly you don't have to go to specific bridal retailers, pale coloured shoes are available almost year round now, tiaras, sparkly jewellery easily found in places like accessorize, Claire's, dept stores even good street markets.
Hair & makeup - you don't have to have a fancy hairdo or MUA, absolutely nothing wrong with doing your own and you actually might feel and look more like yourself if you do.
Depending on venue consider "self catering" - in quotes as you may still use a professional caterer but just not a "wedding" one - do something a bit different, a picnic, retro (cheese & pineapple hedgehogs, sausage rolls, vol au vents), cream tea, BBQ - all kinds of options - we had a hot buffet which at the time was seen as "casual" even though the food was high end (we had a LOT of different dietary requirements & allergies etc to accommodate, this was SO much easier than trying to find even 2 served options that would work). If you'll be marrying 2019, then consider stocking up now on party food and freezing (maybe friends & family can store too?) you could even do "pot luck" where everyone brings a dish (but co-ordinate so you don't end up with a glut of potato salad & dips).
Use a "normal" baker for the cake - it doesn't have to be fancy, you could even go completely removed from tradition and have a cheese "cake" (tiers of various cheeses), or a croquembouche, or a gingerbread house - all things I've seen. One friend who was having a goth style wedding ordered a dark purple "birthday" cake (but asked for no writing) and got gothic topper online. Sadly often as soon as "wedding" is mentioned prices can get stupid!
Flowers - again high street florists are usually more than capable of a few buttonholes and a hand tied bouquet. You don't need to get buttonholes for all guests and bridesmaids don't have to have bouquets either, they could carry a single flower with a ribbon tied round (easy enough to do yourself) or have nice but cheap little bags, or indeed nothing. You don't even need a bridal party.
Entertainment - ask around, not only will you likely save money you'll get someone recommended. I've also known weddings where it's simply been a case of hooking up a playlist to a load of speakers or karaoke so the guests provided the entertainment.
Photography - I would say don't skimp on this as its the only permanent part of it all (aside from the marriage of course) BUT you can shop around to get the best deal but always check their portfolio, choose someone you're comfortable with - they're going to be getting up close and personal at points and you need to feel you can trust them. We had someone new to the profession but who was recommended by a relative who was still building their portfolio, they gave us a slight discount based on us allowing our pics to be used for publicity (slightly themed wedding plus we wanted a couple of unusual shots which he was happy to do but wanted to use them to show how flexible he was).
If you have creative friends & family their gift to you (if they're willing and you don't pressure them) can be a contribution to the event. I was lucky one friend made the flower girls dress, another made the headbands for all the bridesmaids, another did the stationery inc place cards, I made the favours myself (you don't have to have favours - I did tablet which not only got scoffed but motivated many English people to start making). I've known/been involved in weddings where people who were close to the bride and groom have done all sorts inc a 4 tier cake (b&g covered cost of ingredients, baker was happy to give time for free, b&g made sure to do a lovely FB post after with pic of cake and link to bakers work FB page, I believe baker got at least 4 further jobs out of this), a friend her aunt made her veil (she was a hobby lacemaker), another her sister made her jewellery inc the rings so they were bespoke and themed.
Hen/stag - in my day (yes I'm old! 46! 😂) you didn't basically go on holiday of a lifetime for this! That's ridiculous and usually means someone you'd really like there can't make it. We basically just did meal + pub crawl, my grans came on mine and one got completely tipsy and started telling all kinds of secrets (funny not sad or concerning in any way, like she knew which of her kids it was that broke the telly that time), she also surprised us all by knowing not only the words but the dance moves to the then no1! Going to maga with my other 20-something friends wouldn't have been nearly as much fun, nor as moving.
I've just remembered someone I know had a 70's wedding with a maxi dress, hair loose, groom in flares, village hall done up 70's party style, babycham for the toasts! (Which people loved!), 70's food & music. Was fab!
Your fiancés field idea is romantic but not really realistic. Iirc there was a post last summer by an mner who attended a wedding where the couple held the reception in a field - but the couple hadn't taken into account:
Access (was a long way from nearest road) & guests were expected to BYO food and drink - bottles of drink are bloody heavy!
Plus some of the guests were mobility wise disabled! People who need crutches/walking frames on a flat surface aren't going to manage an uneven muddy field 2 miles from the nearest road!
Adequate provision of toilets/handwashing facilities.
There were no baby changing facilities
He's also almost volunteering his mum without considering how much of an undertaking cooking/prepping food for 50 people is! Does he cook much?
"Toilet situation... We could use the house and there is an outdoor toilet already installed at PIL in the outbuilding in the field." That's nowhere near enough for 50 people! They'll get blocked!