My eldest daughter did Maths FM Physics and Chemistry Alevel. She was glad she took 4 alevels but she could have dropped Chemistry as she wanted to do physics. Physics is full of maths, and if you went down this route, the FM really does help for first year and students find the course easier. But you can still do the physics without FM alevel, you are just moving it along to first year studies. If you want to do a joint honours of economics and maths, then dump the physics.
If you are going to do the 4 alevels, be prepared for hard graft. Start making your revision notes as you go along, maybe flash cards, watching videos, and develop a plan to study / revise, but also take a break. Be prepared for year 12 not to be as great at year 13. Eldests tests was coming out at D's in year 12 and like many of her classmates they too was the same, but by year 13 it really improved. Give it until Mid to end of October. If you really do find it full on, then ask if you can drop the physics.