Gap year suggestions for physics.

Honestly there's no need to do anything specific to the field. Getting a random job in a shop, restaurant/pub, or if you're lucky some clerical/reception work in an office are all perfectly good employment options for a gap year - as the key is you begin to build up a work history for a CV and can hence prove to employers for more specific opportunities which are more often targeted at e.g. current degree students, that you actually can for example consistently show up on time and avoid committing gross misconduct, at a minimum level. You'll also have plenty of opportunities to develop relevant soft skills and specific examples of them in terms of e.g. communication, working to deadlines/in a time limited environment, managing difficult situations, etc.
On the academic side I'd just suggest making a point of doing some maths every day to avoid going rusty - it doesn't need to be loads even just one or two problems a day might be helpful. If you didn't do A-level Further Maths in 6th form then either doing it in a gap year or if not, just spending some time self studying some of the material (particularly matrices, complex numbers, and methods of calculus) would probably be helpful for when you start the course

I believe gap year students can do BPhO, although I'm not sure. You could also see if you could participate in COMPOS or Quantum Club.
I think these are the best introductory physics lectures - there are also problem sets for you to do: https://oyc.yale.edu/physics