Why is the UK so cold and wet?

It’s gash. 

Wait until you find out that countries that are further south and not on the edge of a massive ocean are warmer and drier.  

That's just going to blow your mind.

Southern England's late spring summer and early autumn climate is ace - warm but not hot, usually dry but with a decent amount of rain to keep things fresh and green.   The perfect climate for outdoor activities.   In much the world the summer days are virtually a right off (hence the siestas) and nights too hot to sleep.  The price of course is long wet grey dreary mid October until the end of April period but on balance it is a price I am willing to pay.