"In a survey of academics by the Guardian, Warwick was rated together with Cambridge as the mathematics course they would most like to send their children to."
And I would say:
Warwick, Imperial, Bristol, UCL in that order.
LSE is a great choice too if you have a strong interest in economics and quantitative finance.
All would be excellent remaining slots and if you don't want to be in London: Durham, Bath and Nottingham are also very good choices.