The UK has a remarkable ability to attract human capital of all kinds, and at all levels; this is to be expected of a country that has staked a claim to land on every continent, whose language is the foremost for all types of international communication, and despite a relative decline on the international stage over the past century, is somewhat safer from foreign intervention than certain Eurasian countries in the news at the moment. Before anybody says anything about people "stealing our jobs" whilst earning money that is then largely, erm, spent on goods and services in the UK, remember the things that the UK has "acquired" from other countries. There is no Kenyan equivalent of the British Museum containing, say, the Stone of Scone.
London does of course have a larger proportion of foreigners or second / third generation immigrants of different ethnicities; but that is to be expected of a world city, one of only a handful in the world. If people in this country want to have the opportunity to work in New York, or Paris, or Tokyo; then surely it is hypocritical to deny an equivalent opportunity to foreigners. I find it strange how many people who adhere to free market neo-liberal economic principles are all in favour of foreign investment and the movement of capital, but are somehow less keen on the movement of human capital, unless accompanied by a Centurion Card.
In terms of language, there are plenty of foreign students I know with a far firmer grasp of English than some of the ethnic English people I went to school with. And there are plenty of British people in Spain with only a rudimentary grasp of Spanish, one of the world's most used languages.
In terms of culture, I think the flow between the UK and the world has been slightly more outwards than inwards; as demonstrated by the "God-fearing" homophobes of some African states who seem to regard homosexuality as a benign Western influence, whilst conveniently forgetting about the white missionaries who introduced their forefathers to Christianity in the first place...
Of course the UK is densely populated compared with, say, the US, but a lot of the population is crammed into the south east. Liverpool has lost almost half its population since World War II; so why not allow regional authorities to integrate the city with Manchester on a transport / economic level; give more tax raising powers on a local level (reducing LA dependence of central funding), and have less of a gulf between London and the other major cities? Oh, wait, our Coalition scrapped the regional level of planning and government in England...
Furthermore, the UK has an aging population, and unlike British retirees who go to Spain or Thailand; most of the immigrants to the UK are young, and for the EU citizens, typically only stay for a few years before returning home.
So overall I have a generally positive view on immigration, because I wouldn't want anywhere else putting up more punitive barriers to prevent me working overseas.