I'm in a similar position to you, James: I planted about six fruit trees last winter, and am about to order more. They were one year maidens, on MM106 rootstocks, and were between two and five feet tall. They've established well, helped no doubt by the copious rain. I accept that they'll take longer to fruit; I balance that against stronger growth, and considerably cheaper prices (about £12/tree) -- and I can train them to the form I want (half-standards, for these).
I can recommend the nursery I got them from, if you'd like, along with an alternative that can supply a staggering range of apples. I note that you're in Tunbridge Wells: if it appeals to you, there are lots of varieties that originate in Kent, and which might be ideal for your conditions?
-Ian