Well it depends somewhat on what you intend to do after A-levels.
If you plan to do Physics, or Engineering in general, Physics would be required or highly desirable for most courses. There are a few engineering courses that don't require Physics (many Materials and Chemical Engineering courses accept both Physics and Chemistry, and a few other engineering courses such as Civil Engineering at UCL don't require Physics) but they are in the minority as indicated. Of course if you want to do Chemistry or something related, it would normally be necessary or advisable to take Chemistry .
For any other courses (including, for example, Maths, CS, Economics, Law, etc, etc) it would be best to take the one you are best at to maximise your overall grades. If you aren't sure what you may want to pursue, it may be advisable to consider taking both - perhaps in lieu of CS, as CS is not a prerequisite for any course - to maximise your options later on. If you are much weaker in Physics it is best to take the other subjects you are strongest in. It may be worth noting a number of desirable Maths and CS courses express a preference for A-level Physics (Cambridge notably, for both subjects).