Should I do AS Maths for these benefits?

At GCSE I got a 6 (B) and was 1 mark away from a 7 (A). A lot of courses I've been looking at asks for a 7 at GCSE if you don't do A-Level Maths, including Geology at Imperial, Account/Business etc at Durham, and Economics at unis that don't require A-Level Maths.

So, instead of me sending them a sob story about how I was 1 mark away from meeting their requirements, should I do Maths as an AS to make up for it? I don't want to do it as a full A-Level as I feel that I will get a better grade in something else, so if I do it at AS it won't affect my A-Level grades and has half the benefits of A-Level Maths I guess? I'm also doing Physics so AS Maths will help with that as it includes mechanics. Also, I feel that Maths will help me in the real world when finding a job as Maths is such a useful skill, if only I focused on it more at GCSE.

Isn't A-Level Maths linear now though? As in it's a full 2 year course and only has internal exams at the end of year 12?

You can still take the AS as a standalone qualification.

Doesn't it depend on the school?

Some schools don't like people doing them, because they don't want to offer classes for them. Doesn't mean that you can't do it as an external candidate though.