First of all, I don't like London as a city very much which is why UCL was never really on the map for me. I have applied for LSE, but it'll take a lot (i.e. funding/scholarships) for me to actually choose LSE, even if I am accepted.
So I guess the only input I have for you is that you really should consider how much more expensive London will be compared to Barcelona. You're probably looking at as much as €700-1000 extra per month to live in London compared to Barcelona.
Second of all, I'm almost in the same situation as you are. I got an offer for the master's in economics at BGSE (100% tuition fee waiver, to my surprise). The generous offer, however, makes the choice between BGSE and TSE very hard. I also hold an offer for the M1 Econometrics and Statistics at Toulouse School of Economics.
How do BGSE and TSE stack up when it comes to reputation (given my goal of doing a PhD)?
Profile for future reference (following your example):
* Highest grade in almost every economics module I've taken, top 3 university in my country
* Top grade (as of now) linear algebra and analysis in one variable
* Exchange year at top-5 US econ department, straight A's Econ upper division courses
* RA work during exchange and after returning to home university
* GRE Q166, V164, AWA 5.0
* Waiting for decisions: LSE Economics & Philosophy, LSE Economics Oxford MPhil Economics, Oxford MPhil Economic and Social history, PSE M1 APE