It's not hard to find sources that say that the UK punches (punched?) above its weight in the research, and there're lots of people who want to go to the UK for work or study. We can see this in StackExchange questions as well - people often explicitly ask about doing PhDs in the UK.
Why is the UK so attractive to researchers & students? Presumably the availability of funding is a big deal, but as long as the UK is part of the EU, funding levels shouldn't be too different from the rest of Europe. It's possible the UK has high quality of life, but it's not like other EU countries do much worse in this area, in fact several outperform the UK. It's also possible that the UK simply has many more prestigious universities than the rest of the EU, and prestige attracts people; however I find it hard to believe that this is a major factor, especially at postgraduate level and beyond. The only other thing I can think of is that the country uses English as its primary language, but that should be less of a factor in academia since most academics can speak English (it is the language of academic communication after all).