considering a 7 is an A, yes, it's good enough to take the a-level. if you mean you're reconsidering your options in that you want to take a route with maths and physics like, say, engineering, feel free to but it's definitely best to stay with what you're interested in; that'll make it so much easier to read around the subject and think about why you want to study it further, as well as what you might want to specialise in. but that's a long way away! for now, choosing the subjects you enjoy is the main priority.
if you want a sense of what you'll be covering, you can skim through the specifications for each subject. stop if you feel overwhelmed, but if you find that you actively want to learn the things that are outlined in front of you, in one subject more than another, it's a good idea to go for it.