If you got a drive and passion to study those subjects, by all means go for it. But just be careful though; GCSE is in no way like A-Levels. Where you could study summary pamphlets containing formulas in the space of 3 days to hit top grades in GCSE, A-Levels require consistent, full-on focus from the get go if you aim to achieve those high grades you aspire for, especially with content heavy subjects like Chemistry and Physics, probably 2 of the hardest subjects at A-Level, so you could imagine with the new rules implemented how stressful it would be to try remember everything in the space of 2 years for those exams.
At the end of the day, as much of a merit completing 5 A-Levels may seem, universities only really care at the end of the day about the top 3 grades you achieved at the end of the 2 years of your course. No advantage would be given to you over other applicants who have done only three A levels.
Take this for example; one of my friends dropped a subject mid-way through her first year and continued with doing Maths, French and Geography; at the end of 2 years she achieved A*AA and got into Bath to do Economics. Bare in mind she only got 3 A's at GCSE.
One of my other friends got all A*/A grades at GCSE. This got over his head, leading him to do 5 A-Level subjects including Maths, F.Maths, and the 3 sciences. In the first year he got ABBBD with the D coming in Physics, which he ended up dropping. The year after he got ABBC, consequently missing a place in Durham to do medicine. Was it worth it?
Don't get me wrong, I'm not cajoling you into going out of your way to do less subjects than you desire, but life doesn't always work the way we want it to work, and sometimes it's just best making sacrifices to get a better end result than otherwise. All the best of luck however direction you choose tho homie