Question - if you want to do law, and your sixth form no longer offers it as an option, have you considered switching providers to one that offers exactly what you want? In my personal experience, it's better to choose the A levels you want to study. If you're asking whether you should do maths, that tells me you actually don't want to do it and you are hoping someone will talk you into it.
Is maths a good subject to study? Absolutely. I think most people will agree maths is generally a good subject to have, and well regarded by any university admissions, but only strictly necessary beyond GCSE for a STEM subject or accounting type subjects. For everything else it's just a plus. So, is maths right for you? Only you can answer that, but I'd say to only do it if you really want to. Don't feel like you have to settle when it comes to your education.
That said, if you start the year and realise you want to do something different, that's also OK. So you can try maths if you're not sure and then if it doesn't work for you you can drop it next year. I studied maths at college and failed it because I didn't work hard and only took it because I thought it'd look good on my CV. Came out of college with 2 a levels grades D and E in French and Physics. Somehow managed to scrape into university through clearing with a place in Philosophy. Then somehow ended up doing linguistics (in first year they let us pick modules from any subject within the school of humanities and that's what I ended up loving). Did a study abroad year and graduated in 2012 with a 2.1 (just narrowly missed getting a 1st - I got an average of 69 and a 1st is 70) Then I spent 10 years wishing I had done Medicine but figured I would have never got into a medicine degree. Then last year at the age of 32 I decided to go for it, went back to college to get my Scottish Highers (same as A levels) in Chemistry, Biology, Maths and Physics and applied for uni places in medicine. I got AAAB (B in Physics), scored very well in the admissions test (UCAT, top 7% score in the country) and now I'm starting in Medicine at Aberdeen in 2 weeks. I'm not saying any of this to brag, I'm saying even 15 years after I failed maths, it's not too late. The decisions you make now are important because it can decide the direction you take in life, but if you make the wrong decision it's not that bad and can be fixed.
Not sure if this helps at all, but whatever you to I wish you the best of luck.
Edited to add: I was also a super high achiever at GCSEs. The kind of high achiever that didn't even have to put any effort in. You can achieve your A*/A in A level Maths, so long as you put the work in. It's easier to put the work in if you like what you study.