It basically depends on the ideal temp range listed for the yeast in question. Most brewing supply websites list this kind of info. Sites like Midwest also have PDF files that describe starters or rehydrating,etc.
For Ale yeast, the ideal range is around 65 - 75 F (18 - 24 C). You may be getting a little cool in the UK if you don't have any means of temperature control. You are however, right in the range for lagering. Lager yeast prefer it much cooler around 45 - 55 F (7 - 13 C). The fermentation process take longer when lagering, but it is well worth the wait.
I don't think it gets cold in the UK (says the guy who lives where it is expected that the temperature in winter will drop to -40 (that's the same in C as F)). The current temp is 4C. and raining lightly. It's too nasty to work outside so I'm brewin!