Why is the UK such a brain magnet? 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 &amp; 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).
From what I have heard from students and academics in the UK there are a few key factors:

I think you may be undervaluing the effect of language. English is by far the most widely studied language and is the primary language of communication in academia. Students want to improve their English while they study. While universities in non-English speaking countries may teach in English and it is most likely possible to do research in English, day-to-day life can be isolating without knowing the local language, for example see here. It's also much less daunting to live in a country if you are already competent in the local language. Similarly for faculty, its much easier to work in a country where you're already fluent in the language rather than starting from scratch in a new language.

The UK is widely seen as welcoming and accepting of different nationalities and cultures. You could debate the extent that this is actually true, and it may be changing. It is certainly true at least of university cities such as Oxford and Cambridge as well as major cities such as London.

The UK is home to some of the best universities in the world in Oxford, Cambridge, Imperial College are all in the top 10 (the rest being in the US - which also suggests a language effect). These universities are also widely known worldwide. Oxbridge in particular offers a certain prestige which attracts many people, mainly due to its history.

I don't think the UK is necessarily attractive to top academics, especially in STEM subjects. The cost of living is high, especially in cities such as London, Oxford, Cambridge and academics (especially junior academics) struggle to live in these cities. There has been a large push by Oxford and Cambridge to provide subsidised housing for staff in order to attract/retain staff. I know many top academics who have left the UK for the US.

I also don't think the UK is particularly extraordinary in terms of research funding. 

In conclusion, I believe language, culture, and brand recognition are the main factors for making the UK attractive to students and academics. Although this is threatened by the cost of living and rising xenophobia. The outlook for research funding is also uncertain due to Brexit.