If you really want to go to London, take IC.
(ever considered their MRes in Stochastic Analysis instead given you had a master already?)
IC is definitely a well-regarded school in London, and also in HK. The only downside is.......it is expensive.
ETH is a very good school, but the location is the concern.
(don't make me wrong.....it is a very good school, and it has a very strong reputation in EU. If you has a European passport, a job in Zurich is a walk in the garden after graduating there....but if you are an international, it is a tough fight. Look at their placement record, things are not bad, but it is far from safe. What can be said is, a job in Frankfurt or other European city is safe. Her reputation in far east will not be a problem after working a couple of year in a related role in Europe, since at that stage your CV are likely to go to the recruiting team more directly, and people in this subfield of finance all know what ETH is.)
For obvious reason, LSE MSc Fina Math is out of the discussion when talking about this topic.