it’s a tough one. i dont think you can rule out the London vet because he is strict about the age thing; did he explain why that’s his policy? i know that the lady at my local pet-shop who’s ‘qualified’ in these matters (though she comes from a strictly academic pont of view- she believes rabbits are too smelly to be house pets) told me i should wait for a few months for Lollo’s hormones to settle which would have taken him up to about 6 months. having ignored her, i have no doubts i was right, but each situation is different and i think the vet should be willing to look at each case on its merit like mine did.
my vet has her own rabbits so when i phoned up to enquire initially the nurse spent a lot of time explaining how good harriet is specifically with rabbits and the sorts of things they deal/ have dealt with. this really helped.
if you haven’t found a more local vet that you trust he may be the best option BUT i think its a good idea to find a local person you can trust and that can build up a knowledge of your bunny so that if there is an emergency ever you know they are clued-up and have the history to hand.
the (whichever) vet will do an initial check to make sure bunny is fit enough to go through the operation; i’m sure once you’ve made the decision the initial meeting will put your mind at ease at least as easy as it could be in the circumstances!