Like the above poster says, it's a requirement to have a C grade. Getting a D won't make your application look any better, they simply won't look at it at all because it hasn't got that magic C grade on it.
You can moan all day about how the QTS tests are unfair, how you won't ever possibly find a use for maths in your subject, but the fact is you are going to be faced with numerical problems to solve in your career and the tests are a fair standard to make sure that everyone's on the same standardised level when starting training.
Personally, I would've focused on the maths GCSE in your AS year, as you would've also had another extra year (A2) to try again if you fail. Get a head start on it now and start revising for next year, build up every bit of knowledge you can. Look at mock exam papers online, ask people for help that you know are good at maths.
Once you've got past the hurdle of GCSE maths, then it's on to the next hurdle of your QTS tests. The only thing to do for this is to practise, practise, practise! I was like you too - I was getting 4/28 on the first practice tests I was taking. You just have to keep going back to the tests, work out what keeps cropping up (i.e percentage/decimal/fraction conversions etc) and work on your speedy methods for those. If you're doing this alongside your GCSE though, it might come quicker to you as you'll be learning maths anyway at the same time. Take it back to basic level if you have to. I was referring back to a SATS maths revision book when I started looking at the QTS tests.
If you're going to be teaching Secondary, I assume you'll be doing a PGCE...in which case, don't worry about the QTS tests for another 3 years until you've got your English degree first! Just concentrate on your GCSE!
Good luck with next year.