I would definitely suggest going to UCL, for the following reasons:
UCL seems to be vastly better than Bayreuth, as quick searches on world rankings suggests. Though this method has flaws, I think it is pretty telling that the Shanghai ranking puts UCL at 21st in the world, and Bayreuth past 400... it also suggests that it is internationally recognised, which is a massive positive that I don't think you will be able to get elsewhere. I may be wrong, but I definitely think having a 'UCL Degree' will be much more valuable than having a better grade degree from a lesser university.
Next, London is a fantastically cosmopolitan city, so living and studying in it would be an experience unlike anything else. Also, being in London, you would have access to the various graduate jobs fairs put on locally, which have a reputation for being targetted by finance firms, city jobs, etc, though I can't verify that personally!
Most importantly, language. It seems to me that your main concern is with your English, but I don't think that's too much of a problem? Living in the UK would rapidly improve your language skills, and I believe that the university has a language learning centre, set up to ensure that everyone has the skills they need to complete their course. See here:
http://www.ucl.ac.uk/clie/english-for-academic-purposes I get the impression that if the university want you enough to offer you a place on such a good course, they will invest time into you so that you can complete it! So, my opinion? Don't worry, come straight over to UCL and get started on your work!