Another A-Level in gap year

I think it's possible. Would you be taking exams as a private candidate?

If I were to take a gap year and take another A-Level (a subject I didn't do initially) would I be able to do that A-Level in one year?

This is what I'm doing. I've decided that I want to do Economics at university and for every RG London Uni I need A-Level Maths as an entry requirement. I finished college this year and did Psychology, Econ and Comp sci.

The only college (In London) I found that does one year A-Level courses for free as long as you are aged 17-19 is Westminster Kingsway which I've applied to and have my Maths Entrance Exam on the 31st of August.

I've called up most of the Top Uni's and asked whether this would affect my chances of getting in and the only answers I received were:

1) No
2) Slightly decrease my applications' competitiveness