it's not too hard to see the cons - poor, rainy weather, which can be depressing in itself. Old, mouldy houses with poor thermal insulation and generally high cost of living. Traffic in towns and cities. Uptight, insular, class based society that is not exactly welcoming to foreigners.

Now if you swap UK with London, a lot of these cons go out of the window as your neighbour is probably also a migrant. You still get very high cost of living, very bad car traffic, but relatively decent public transport. Wages, given the cost of rent are not that impressive and the weather in London is a bit better than the rest of the UK, but could be better, of course.