The new A-levels are linear, rather than modular. AS levels exist but are a separate qualification; if you take the AS exams in year 12 for example, these results will have no bearing on your final A-level results as you'll be taking a full set of exams at the end of year 13 anyway. It's more similar to the IB style, where you may take mock exams in year 12 but all your actual exams will be at the end of year 13 and will cover the entire course (or significant amounts of it). The benefit of this is there is slightly less emphasis on "learning for the exam" and more on just mastering the complete syllabus. You won't be able to as easily engage in "targeted revision" and skip topics you're not interested in as they only form a part of a single module which averages out; you can't know if it will come up and if it does, how extensively it will be examined.
If you didn't particularly enjoy physics at GCSE I'd probably suggest avoiding it at A-level; it's really "more of the same", fairly unsophisticated mathematically and more of a "survey" type course. You may want to consider Further Mathematics A-level, and taking mechanics option(s) if you would like some physical background and have a good aptitude and enjoyment of maths; the mechanics content of Maths/FM is more mathematically sophisticated and closer to what you may encounter at university, and while less broad is thus more representative and interesting, in my mind. The main topic at A-level physics which isn't covered by the mechanics modules which is not completely developed from scratch at university usually is electricity/magnetism; in particular the basic circuit laws. This isn't universal however so do take that with a pinch of salt.