I wouldn't care that much about the acceptance rate. When it comes to LSE, there are many people who apply, then they are accepted, but then they're not taking a place. 25k it's loads, and not many people go for it, and thus, this results in the acceptance rate super small. Similar for the Imperial.
When it comes to unis, it's really hard to say, but from my experience employees value international experience, and therefore, if I were you, I'd go for Masters in the UK. Both LSE and Imperial are really good for starting a career in Finance, so I guess it's just a matter which one you prefer. LSE has a longer tradition of putting students into Finance, however, Imperial is getting better and better at this.