It depends by what you mean by work in Wales and England.
Are you living in Wales in a welsh-speaking area, speak Welsh, and want something current but that would not be mangled, or are you in England but would like something Welsh and aren't too bothered about popularity or the name being dated or getting anglicised?
You will get suggestions that are very dated, or are misspelt (usually -wyn on girls' names) or are not welsh (Rowena, Meredith), or that will be virtually unpronounceable if you don't speak Welsh.
If you don't speak Welsh, I'd opt for something like David, Daniel, Martha or Elizabeth and use a full-on Welsh middle name.
From your OP, I'd say that you do speak Welsh, and your suggestions are fine, but Gwyn and Gwen will have the vowels interchanged quite often (Gywn, Gewn), and will be assumed to be of the opposite sex
.
Macsen will get Maxon, and people might get sneery about it being chav (pah!)
I have worked in the Welsh borders a fair bit, and colleagues sometimes had welsh names, so have heard the anglicised forms. Arwel being called Ah-wul, Huw Hoo, Dewi Dowey, Eirian Erry-un, Mari Mahri/Maree, Ceri Serri/Surrey/Cherry/Cherie, Rhian Ree-un, Ree-Anne, Eleri Ellery/Ell-Airy etc.
It also depends on your surname.
You could use a welsh form of your surname but only if the whole name is very welsh.
My usual suggestion when I haven't NC is
avoid:
C or G before an i, e, or y
Vowel combinations like ae, au, ai, ei, eu, ew, ia, ie, uw, wy
Letter combinations like ngh, si, ar
The letters dd, f, ll, rh, ch, u
Names ending in -an, -en, -a, -yr, -ir, -o and -in, and sometimes -wyn and -wen.
It will narrow your list considerably.
The 'welsh name' web pages usually contain some bonkers suggestions.