Keepers Nursery. Article in the RHS magazine a few months ago - the owner really loves his plants, and was helping his dad graft trees while hs was still a schoolboy.
Frank P Matthews
R V Rogers
Those three are established fruit tree nurseries and were the three listed in a recent RHS article about plums
Ken Muir started with strawberries and has branched out - no idea about the quality of his fruit trees.
More recently to the scene -
Pomona
Cox's Orange Pippin (which is the go-to place for information about individual varieties.
No personal experience of any of these, sorry. My apple trees all came from Scotts of Merriott - they decided to have nothing to do with the internet when it arrived, and went bust a few years later.
You're usually advised to grow apples as an open bush. Pyramid may be more the shape you need
www.pomonafruits.co.uk/blog/growing-pyramid-fruit-trees/
Yes you'll need a dwarfing rootstock - most of the places I've suggested will have guides to rootstocks. Apple rootstocks I think are other species of apple, pears are usually on quince.
Pear trees are naturally more upright - easy to distinguish naturally grown apples and pears by their shape.
Pollination - you could always hedge your bets by growing a family tree - two or three compatible varieties grafted on the same rootstock. Very restricted choice of varieties though. In theory you're right about the crab, in practice I've never had to rely on it.
If you're only having one apple, see if you can get somewhere where you can taste different varieties - there's a lot more to apple taste than sweet/ juicy/ crunch. My favourites are the spicy tasting ones, so I grow Ashmeads Kernel, Cornish Aromatic, Herrings Pippin