I assume this has something to do with the fact that it is very easy for Britons to go to wealthy, English-speaking countries, and also that there's a relative lack of migration opportunities in Britain. If you're American or Australian, you can always pick up and try another city, but in Britain, you mostly move to London or you . . . move to London. This is an exaggeration, of course, but there's nothing like the ability to say, "You know what, things aren't going so well in Boston, so I'm moving to LA." If the economy, or the job opportunities are bad in London, they're probably bad everywhere else in the UK too.
Naturally there's also the fact that Britain's a crowded island where things are very expensive; an engineer can instantly boost his standard of living quite a bit by moving this side of the pond. Standard of living is not everything of course (which is why they aren't all here), but it's something, and people who care about it will move.
Apparently "record" numbers of skilled professionals are leaving Britain. The obvious crack is to say is: who can blame them, given the miseries of the Blair/Brown years? (Not economic misery of course, but the gawd-help-us gruesomeness of New Labour...) But it's also just the continuation of long-established historical patterns: Britons have always had a penchant for escaping the confines of this misty, water-logged island. How else were Australia, Canada, New Zealand and large parts of the United States settled? In my own family one grand-parent grew up in India while another spent 30 years working in Malaya; my own parents spent some time living in Italy before returning to the UK.
Then again it's also the case that Britain attracts an awful lot of foreign skilled workers too. So much so, in fact, that London is now one of the largest French cities on the planet. In that respect much of the UK (and London in particular) exists in a post-national, globalised environment. This seems to me to be a pretty nifty thing. (And a tribute to the EU of course.)
It's true that the grotesque Londonisation of Britain has been just as damaging as it has been rewarding. What's good for London - including its massive public subsidies - is not necessarily good for the rest of the country. Just as it would be good for the United States if one centre of political/media/economic/cultural power were located in the heartland, so it would be useful and, indeed, healthier if London were not the heart of all such efforts in Britain.
Still, it's not quite London or Bust as Megan seems to believe. Quality of life (as distinct from "standard of living" matters too) matters too. It's a small island for sure, but it's still true that there's more to it than London Town. Anecdotal evidence supports this: many of us know people who have relocated from London to either the great English provincial cities (Manchester, Leeds etc etc) or to Glasgow or, especially, Edinburgh. This includes a fair number of Scots who moved to London for work but who have returned to their native heath.
In fact there are some indicators that elements of London's hegemony are being challenged (at long last!). The BBC is, quite sensibly, relocating Radio 5 Live and other services to Manchester. Meanwhile, in the business world the Royal Bank of Scotland Group (one of the world's half dozen biggest banks) continues to thrive despite being headquartered in Edinburgh not London. Folk would have laughed at you 20 years ago if you had said that it were possible for a global financial services company to thrive without being based inside the City of London. But there you have it. RBS's success is obviously good for Edinburgh and Scotland, but it's also an inspiration.
In fact Scotland - despite it's often depressing politics - continues to do pretty well. More than 50,000 people relocated here from other parts of the UK last year. That's equivalent to roughly 1% of the Scottish population. By some estimates there are nearly than 500,000 English people living in Scotland. They're our biggest "minority". They're not coming here for the climate, I guess.
So an awful lot of them are coming because Edinburgh (and Glasgow) is just a nicer place to live than London and the crowded south-east of England. This is obviously especially true if they've got children and are looking for somewhere affordable to live. London's crippling expense is Scotland's opportunity (also an opportunity for the north of England). Though internal migration may not happen at US levels, it's more common in the UK than people think. If it were ever true that it's London or Abroad that's less true, in many sectors, than it was.
London's a great international city of course. But it can't provide everything. People looking for a better quality of life, even at the expense of a nominal pay-cut can still find plenty of opportunities elsewhere in the UK and especially outside the clogged, claustorphobic south-east of England. (As for the idea that if the London economy is poor then so is the economy elsewhere: not so; in the early 1990s there was no recession in manufacturing or in Scotland until the need to cool the over-heating London economy put the rest of the country into recession. We were last in, last out - one reason for favouring whatever independence means these days...)
So, sure, lots of people want to leave London because, for all its treasures and dynamism, it's an often ghastly, impossible place to live. But that's fine. It creates opportunities for "livable" cities such as Edinburgh or Manchester to take advantage.