If you approach unis about your circumstances, they may well say that yes you can do the maths stream but you may need to do some extra 'knowledge enhancement' course too, or they may even say you're A-OK just to go along with the graduates of straight maths.
The bursary for training to be a maths teacher is £25,000 if you get a 1st in your degree and £20,000 for a 2:1 (last year's figures, but fingers crossed they'll stay the same).