You meet (in fact exceed) the standard requirements, so I can't see why there would be any issue. Believe it or not, Oxbridge admissions computers are not simple machines that look at your number of A*s and decide whether or not to give you an offer on that basis, and the process is fortunately somewhat more sophisticated than that.
In terms of whether to drop chemistry it's up to you, continuing chemistry as a 4th subject and getting an A in it is perfectly fine and isn't going to harm an application (provided you are confident you can do well in your other subjects while doing so!). The only thing to consider is how your school teaches/arranges the exams for maths and FM - if you do all of A-level Maths in year 12 and do the exam then, and all of FM in year 13 and do the exam at that time, then Oxbridge may prefer you continue with the fourth subject so they do see at some point you are sitting a "full set" of exams for 3 subjects in a single exam session.
Regarding GCSEs, for maths specifically at both universities I gather they're rather secondary as a consideration. This isn't necessarily true for other courses there but I've seen it from fairly authoritative sources on TSR that GCSEs are less important than other factors (e.g. the MAT for Oxford, or for Cambridge the interview as they will generally try and interview you if you are likely to meet the standard offer - unless you apply to Trinity College in which case you have a good chance of being rejected pre-interview regardless, so just don't apply to Trinity for maths...), and the Cambridge ATs that do/have posted on here in the past have indicated they haven't previously rejected an applicant for any course solely on the basis of GCSE grades before.
Essentially you are perfectly fine. Focus on deciding between the two courses (which have some subtle but important differences) and crucially on the
very different admissions process for both. For Oxford you do the MAT
before you are interviewed and it will be a major factor in determining if you get shortlisted for interview I gather. For Cambridge you do STEP
after being interviewed and so a higher number of applicants are interviewed and made offers, but STEP is very hard and about 50% of applicants miss their STEP requirement! There's also differences between the two universities as a city and whatnot to consider too