I'm wondering what sort of apple tree(s) to buy.

My garden in Gloucestershire has an 8' wall facing SW. It gets afternoon
sun. I like the idea of planting against it and perhaps pruning it to
keep it below the top.

I enjoy english grown Cox in season and occasionally find a nice acid
Granny Smith in Tesco. Spartan's are quite good.

A variety that would give me fruit when the shops only have insipid
imported apples would be excellent.

So the questions?

1. What variety should I get?
2. Best planting time?
3. Do I have to have at least 2 trees to make pollination work?

TIA!

Steve Harris - Cheltenham - Real address steve AT netservs DOT com