I think the situation in UCL would be very similar to LSE. Even if you are a stellar student, the faculty might want to push you elsewhere, this also contributes to the prestige of their program. But if you are confident that you will perform very well, I will not worry that much, you can always stay from the beginning your interest of staying, etc.
The answers to your other questions are pretty much idiosyncratic. I have a very positive opinion about LSE and London, and I guess the experience in UCL should not be a lot different. Whether this outweighs the risk of not getting in directly or the opportunity cost of going to CEMFI is something I ignore. Btw, CEMFI is also an excellent department, especially if you like metrics, they have Arellano.