No A Level subject is compulsory. The only exception being if your school makes you take one or two particular ones, such as General Studies or Critical Thinking. The minimum number for most (if not all) UK universities is 3, but it is somewhat advisable to take four if you can.
As for which subjects are easier and which not, it's really down to your strengths and weaknesses... Perhaps look around a few specifications and past exam papers of the subjects you're interested in to get a feel before you make a decision.