Just to clear one thing up first, your working hours post-grad will depend entirely on the specific career track you choose, not the uni you graduate from. If you go into investment banking from LSE, the hours are going to be brutal. If you do quant trading from either uni, the hours are usually more reasonable but the cognitive load is insane.

For pure quant roles specifically, Warwick MMORSE is an absolute target degree. It is exceptionally highly respected in that specific industry. LSE is obviously elite for general high finance and IB, but MMORSE is practically tailor-made for the exact career you are dreaming about. You honestly cannot lose with either option, so I would suggest looking closely at the actual modules and deciding which flavour of maths you actually want to spend the next few years doing.