I know this is a bit late but just doing an initial browse of the forum & thought I'd share a few recommends from S.Wales & South West areas.
ciderappletrees.co.uk/ - Somerset sort of area - had some cider apple trees off them last winter (M25 standards) - lovely folks. Regular apples too.
I'd previously got trees from John Worle near Hereford, a lovely old nurseryman but who has since retired. His son, James, is, however, continuing and is based near Yeovil -
https://appleorchardsolutiontreenursery.weebly.com/John was always very accomodating to my small requirements even if his main focus seemed to be bigger commercial cider orcharding (much as bush trees)
I always meant to check
https://www.tomtheappleman.co.uk/ as he has a good few interesting old varieties - guess I haven't used him yet as my focus to date has mainly been on M25 standards (with stem builders) for an old traditional cider orchard.
https://www.welshmountaincider.com/ are LOVELY up in mid Wales!
https://iansturrockandsons.co.uk/ have some amazing, specialist Welsh fruit trees! My brother planted an orchard with trees mainly from Ian. North Wales, I believe..
And finally, near Monmouth -
http://www.carrobgrowers.co.uk/ - Caroline & Robert Boyle have some really interesting plants/shrubs/trees if you're in that kind of area.