Serbia will no doubt be assimilated by the EU relatively soon, but not soon enough to be immediately useful to you.
You are right that there is a good "brand image" for maths education in E.Europe / Russia, but that does not translate to it being a good place to start a career. We tend to call it the "Wimbledon Effect", where the British tennis championships are the most prestigious in the world, but are never won by Brits.
There are some options of questionable validity...
You can come and do a masters in something (not finance) in London, that gets you in, and give you a good shot at ducking and elbowing through.
The reason I say in your specific case not to study finance is that UK MSc finance courses are quite expensive, and I get the impression you don't have serious cash lying around. But courses in statistics, econometrics, et al are relatively cheap. (note the word "relatively)
There may exist an option where a mutant version of the CFA counts as a masters degree for this purpose, but it looks so dodgy I need to dig a bit on this.
It is very rare that my broadcast advice recommends a specific course, but I think you should consider the Master of Advanced Studies at Cambridge. There are those who claim it to be the best course for an aspiring quant in the UK, even though it doesn't actually contain any finance. It's relatively cheap, but hard to get onto.
Lastly, there exist schemes by which you could come to the UK as a visiting academic.
Ironically, I believe that I could hire you as my children's nanny tomorrow, which would allow me to deal with my genius 9 year old pestering me to teach him random matrix theory, which for various reasons I never did myself.
(I hated doing LA)
You might have noticed that I regard these rules as illegitimate and racist, and thus will expend effort to help people dodge them, bounded only by the need to remain within the law.
