I have the 70cc airsal kit (with 10mm wrist pin), a 15.15 Dellorto SHA, 22T rear sprocket and the techno estoril pipe on my A3 and I get 40+mph (64+ kph) with plenty of hill-climbing ability.
It's a relatively affordable setup through treatland, but I dunno about shipping to UK.
I use a #70 jet and advanced timing. Amsoil at 80:1 will help with cooling given the advanced timing. Probably a good idea to get a temp gauge to keep an eye on things. I also have a needle bearing thing for my wrist pin, can't hurt.