I have said this before and will say it again that the best way to learn a language is to interact with the native speakers of that language. Same holds true for English as well. Having had the opportunity to interact with several professional  linguists and language trainers over the years, I have come to the  conclusion that the most important thing needed to  speak like a native  speaker is to interact with the native speakers of that language.

No  school or training can give you an advantage in learning a language  like staying and interacting with the natives does. I say this from  experience of learning from the natives and being multilingual and  fluent gives me a certain understanding of what I am saying.

Languages  like English, Spanish, Arabic, French, etc. also have the distinction  of being spoken in several parts of the world. That means that you don't  really have to travel to UK to learn English the way I recommend. You  can do so by being in the US, Canada, Australia, etc.

Obviously this is just one of the ways of doing what you asked, and I am pretty sure that there are several other effective methods available as well.