Some sixth forms/colleges go through the pacing slightly differently, for example in Year 12 at mine we complete all of normal maths as well as first year pure further maths, and having partially completed some of Further Mechanics and Further Y2 Pure. Granted, I assume the specific college/sixth form you intend to go to do only normal maths (which I believe is what is most common?) Though there are other places available with a different pacing that you could potentially look into, if interested.
I did OCR Additional Maths at GCSE and it, + GCSE Physics, pretty much covered all of Year 1 Normal Maths, and also parts of Year 2 (e.g. Numerical Methods), granted doing OCR MEI B not Pearson as an exam board, though I would assume it's similar for Pearson.
It's worth noting that if you like and are good at Physics, you could replace Modern History with that if you really want to take Further Maths and make the workload significantly easier. A-level Physics covers all of Normal Maths Mechanics and a lot of the Further Maths Mechanics topics, and I find that they do a good job in helping each other, as you look at the problems from slightly different angles. Granted there are of course other topics in Physics outside of Mechanics and Further Mechanics, e.g. Electricity, Particle Physics, Waves, but it might still be worth the consideration.
In terms of whether you should drop Further Maths -- I think at least give it a go for a half term, it's easier to drop a subject than it is to take a new one and catch up. See how it goes before deciding. Across our further math classes I know plenty of people that did well on Normal Maths but bad on Further Maths, and stuck with it purposefully so that they could get through all of Normal Maths within 1 year, drop Further Maths now that they fully know their predicted grade for it isn't as desired (granted they already thought that would happen) and then be much more relaxed in Year 13 as they've effectively only have 2 A-levels (excluding Normal Maths revision and making sure they don't forget anything).