Pete it all sounds fine to me. But I guess there is a miss match in your expectations.
I am assuming/hoping you ask her about the things you want to do, before committing to them. Not assuming she will be happy to parent alone when you want to go out/away? If this is the case I do not think you are being unreasonable.
If you do not check with her first then I think you should.
However, I feel here is an issue...
"My wife has almost no social life as it's never been something she has been particularly interested in. She'd always prefer to stay in rather than go out, so there can never be any reciprocal long weekend away for her (which I can see might grate) ..."
I feel your wife is missing out, have you encouraged her to do more. I know it is her choice but I feel long run this will be bad for her. Too isolated and when your kids are older and need her less (proportionately) she may struggle more to adapt, IMHO.
"...but does that mean I should never see my friends?"
No, of course it does not mean you should never see your friends.
I do think it would be worth having a really honest conversation together about how she would like all this to pan out. Does she simply not want you to go out or away at all? Would she like to do something but feels anxious or scared. Having kids can do that.
I think she is doing herself a disservice by not developing some kind of social life where she is. Whether it is a book group, writers circle, running or walking club, evening classes, anything.
It's also really handy as the kids get older to be friends with their friends mums (and dads).
(And Pete despite being warned that you will get lots of anti-men answers on here, you have not! I am a woman, a feminist and my husband does running, and I don't think you are being selfish based on what I have read so far!)