It's a good university for sure, certainly not a bad one; you'd get a great education if you went there. It doesn't stand out particularly as a university for mathematics, at least as a mathematician I haven't heard of/talked to anyone from the department.
Good universities for maths that I can think of, in no particular order: Oxford, Cambridge, St Andrews, Edinburgh, York, Strathclyde, Bristol, Glasgow, Bath. I've certainly missed a few, but these are some places that I know have good maths deparments (and aren't in London).