The weather in the UK is extremely variable due to lots of factors.None of my friends there ever put much faith in the weather forecast(compared to here).
On one month-long July trip, it started off very much on the coolish/rainy side(which was a relief to me coming from HOT and HUMID New York) and halfway through the month it turned hot and steamy.
The same thing can happen in the cooler months as well(well not hot and steamy). I have spent lots of time there in November, December, January and had lots of what I consider balmy weather, as well as
rain(but rarely all day). Since I am from the NE, it always seems much warmer to me than at home. I have seen snow (rarely) but it always disappears very quickly. I always get a chuckle over the weatherman saying on TV...there will be a frost tonight.....as we have frost every night in winter!!!
It is a much more moderate climate than the Northeast and North Central US and I imagine most of Canada. If you are from the deep south, you may regard it as cold in winter, but I consider it
just right!!! I don't go too often in the summer
but the last 2 years have been extraordinarily rainy causing awful flooding in some areas. That said, I managed to be there in July 2008 for a few days of hot weather(I think the only ones they had) which was very uncomfortable.
The moral of the story is that you rarely have extreme weather in the UK but you should always be prepared for whatever comes down the pike. You will not find everything air conditioned, but you rarely need it anyway. If you are from the hotter parts of the US, you may find it cold in winter but I find it more damp than cold and always bring my cold weather gear!!