Don't know how cold it was but my girlfriend had the romantic idea to see a deer's breath on a cold, crisp morning so we set off to Richmond Park. Anyhow, we got there and and could just make out the shape of a deer or two looming from the surprise 7am freezing fog, we think they were deer anyway . By this point my thumbs and fingers were in the early, most painful stages of frostbite. Luckily, said partner was wearing mitts and after a fair amount of twining from me agreed it'd be better all round if I were to have them for the duration.
It's never too cold to cycle if your partner has the right gear and you're shameless enough to beg.