Your GCSE's are perfectly fine, possibly for Oxford even.
For Natural Sciences, make sure that you have very good UMS, a very good personal statement, good Year 13 predicted grades, good AS grades if you are taking AS exams, and also prepare for your interview. Things look good with the fact that you have A*s in all of the sciences and maths, so you are most-likely capable of achieving well at AS level and therefore should be able to easily get high predicted grades for Year 13 as a result.
It is rather similar for Physics, except that you won't have to worry about your UMS points. Personal statement, predicted grades, AS grades, and interview performance are what you need to focus on.
To show your interest in Physics: read scholarly articles, subscribe to magazines, read physics textbooks, go to weekly physics lectures, join/start a physics society at your school. There are many more activities you could get involved in, but reading outside of the subject and reading/quoting scholarly articles are most favourable. All of these you can talk about in your personal statement, along with a paragraph on a particular topic of interest.