I think Warwick, Imperial and Durham would be next on the list for math, assuming you want to stay in England for a three year degree to keep costs down. But I don’t have any personal experience as I didn’t apply anywhere else (I skipped a couple of years in elementary and high school so I was applying at 15 and would have reapplied the following year if I hadn’t got in).
London will be much more expensive than any other location you might choose, the accommodation prices potentially make it comparable in total cost to a four year Scottish degree, and a very different experience, as a big city which is not dominated by its university like Oxford, Cambridge, Durham, etc. Warwick is very strong academically but perhaps not so appealing (architecture and location wise) for Americans.
My S applied to other London universities (for PPE) but was never particularly excited about those options (though finding out in early November about KCL made for a nice safety). He couldn’t see himself going to somewhere outside London and didn’t apply to any of those universities, and in the end he stayed instate (we are in CA) because its very hard to find a better deal than UCLA and UCB.