Probably the most important thing is that you're happy where you're going - your going to be spending the next 3 years of your life at uni, and your time will be a lot more enjoyable if you actually like the place where you're studying.
I don't know which of the unis has a better reputation for maths or in general - your best off looking at the leauge tables published by the papers for that.
It will be expensive living away from home in London though and will cost a lot more than living away elsewhere - maybe your parents would be happier if you went to a uni outside of london where accommodation and living costs are cheaper.
Living at home isn't all that bad, I know lots of people who do it and get on just fine - yeah you will be less independant - but your saving a lot of money as well - you need to weigh up the pros and cons of both situations.
Have you visited the two unis yet? the best thing to do is get a feel for them and decided which one you think you will enjoy more.