I am currently studying CS with maths and there are quite a few girls in the CS course. In the CS with maths course, around 40% are girls. I go to a different uni than Queen Mary's but I highly doubt you will be the only girl.
And yeah, at A level there was only one CS girl in my year too, but it gets better at uni. and as others said, even if you are unhappy with the gender ratio in your course, you wont be spending all your time in class.