It's a bit of a meme that the UK is always rainy. It isn't always rainy, it does rain a lot but it doesn't rain anywhere close to all the time and when it's not raining it can get quite hot, quite unbearably in fact because we don't have air conditioning.

Anyway heat isn't really relevant to solar panels, what they really need is just sunlight in general, and we get plenty of that even if it isn't particularly hot. In fact solar panels don't actually like being particularly hot so they probably won't work very well in the Sahara desert. Despite what everyone may think.