In December it will be colder than you are used to, can be windy/rainy too. Not a fan of Spain in the winter! But I haven't been there in years.

Where abouts in the UK will you be?
Manchester, Edinburgh, London. The point of the trip overall is to catch up with DH's family & friends. I'm a bit scared how cold it will be. Last time we were there was early May and I got really sick. I am really crap in the cold but I'll invest in some thermals.