In reply to Owen W-G:
It's all a bit of a sliding scale really. badoom tish!
For the average climber in the UK, what you really need to do is work up some sort of conversion that includes air temperature, wind speed and sun intensity all together because any one can really bork the equation.
e.g. I find I can climb down to about 2 or 3 celcius so long as there is no wind and full sun somewhere between april and october but equally I could find that I can't keep circulation in my fingers at 15 degrees when there is a 30mph wind and the crag is in the shade.
Purely on the wind subject, for me:
10mph - light breeze, very pleasant.
20-25mph - fairly windy, not great for balancy routes but definitely something that can be put up with if you are keen
35-40mph - Clinging to the cliff to stay on, maybe only moving between gusts, not really climbing weather.
70-80mph - crawling only near cliff edge, very hard to set up anchor, can't let go of abseil ropes, dangling over Screaming Geo becomes seriously worrying.