Visas will depend on what passport you hold. If you have a non-EU passport, you will probably need a visa or a visa waiver to enter the UK. The Schengen area covers most of the continental EU, and again for a non-EU passport you will have to apply for a Schengen visa for the country you're entering the EU in; once this is granted you will be able to spend up to 90 days in the Schengen area without a further visa (but as you will be leaving the Schengen area to travel to the UK, best to check this with your local Spanish embassy or consulate!). As for weather, Northern Europe is basically mild and wet between April and September, but there can be surprises either way - this year we've had freezing temperatures in May, followed by a summer of temperatures in the mid-high thirties centigrade.