I go to school in Canterbury and Kent Uni has quite a nice campus with very good facilities, not that I know a great deal about the maths course specifically. Canterbury is also quite a good city area but it's worth bearing in mind that transport wise, it isn't brilliant and with at least 5 schools around the city centre there are always school children EVERYWHERE and it's not very nice! Royal Holloway is my top choice and it's clearly the opposite town wise but I would assume that out of the three, Royal Holloway would be the best. On my Physics visit day I met a girl who was applying for Phys&Maths and she said the maths dept was amazing.