Hey, I’m a year 12 student studying maths, further maths, computer science and economics.
I recommend you further maths.
Further maths is harder than physics content wise (in year two and extending modules), however it’s easier to get a better grade as studying for it is just practice questions. You will cover Pure further year 1 and 2, aswell as TWO extra modules throughout both years (The choices are between Futher pure 1/2, further mechanics, further statistics and decisions)
Physics is a great a level to have, however it’s only really amazing if you plan to go into engineering. Wheras, further maths is kind of seen as like a bonus on top of your of your A levels. If your applying for competitive universities like Oxbridge or Imperial, they both either require or really really (really) want it for courses such as finance, mathematics, computer science, etc
Further maths and A level maths (of course) have great synergy. It would even seem at some points that it’s just one big a level.
I can’t tell you much about physics apart from everyone I know is struggling in both year 1 and 2
Conclusion if you can’t be bothered to read allat lol: FM is easier to get a higher grade in, more beneficial when going into further studies (unless you wanna do engineering) whereas physics is way harder of the rip