Further than going to say it's your choice, I would actively encourage you to drop one. For maths, neither Warwick nor Cambridge will mind much if you have 4 A2s or 3 A2s. Seriously STEP is so much work that I totally regret not dropping Chemistry straight away. Ended up with a D in it, got in to Cambridge anyway, but I missed my STEP offer slightly and got let in because I was borderline, not because I made the offer officially. Those lessons you have and the work that's set for the subject you want to drop; even if you don't spend much time on them, take away precious STEP and maths revision time.
Furthermore, Physics might help your application slightly, but at the end of the day it doesn't matter too much at all, and people do all sorts of third subjects along with M/FM when applying for maths. What matters most is that third grade is something you can get an A without detracting too much from A-Level maths and STEP. In particular, Cambridge would much prefer to see mechanics modules in mathematics rather than Physics A-Level, so if you can, jam a load of mechanics in there and drop Physics like it's hot. If you enjoy Chemistry more, and have done well in it, you're more likely to get an A in it next year, and it's still a very respected A-Level subject with regards to being a third A-Level.
For course like NatSci and Engineering, Cambridge do like to see Maths/FM/Phys/Chem all the way to A2. But Maths is different and they don't mind so much. If you were applying for those subjects, your application would be hindered if you were to drop either Physics or Chemistry. This is not the case for mathematics in my experience. The colleges do not mind whether you have 3 A2s or 4. They want high marks in Maths/FM and 1,1 in STEP along with an A in your third subject. I would suspect Warwick maths has a very similar policy.
Edit: And I would also recommend dropping Physics or Chemistry (whichever one you decide) before you apply because it's a pain calling Universities up in the middle of the year asking if you can still have the offer even if you're doing 3A2s rather than 4.