Won't ride on the road if there is ice about (fallen off and hurt myself doing that), offroad is still fair game though. I'll quite happily ride in temperatures down to about -5C but as it very rarely gets stupidly cold in the UK I've not really had to test myself further. My nose does run and fingers toes take a while to warm up but usually do.

If its over 30mph gusts then I definitely won't ride on road but I'll still generally be up for offroad unless its raining/snowing horizontally to go along with it. Rain (without strong winds) is generally not a problem, if you don't ride in the rain in the UK then you won't ride much!

I'm rubbish in heat though - I think about 25C is about my limit, doesn't often get hotter than that here. Ideal is about 15 - 20C sunny day with a little breeze.