Indeed, though there is a financial element even for the middle class.
Speaking from experience, there are several reasons for British emigration:
* Older people often move to Spain, Malta, etc. to enjoy the better climate in their retirement (or before). I don't know precisely how big a portion that is, but it's non-trivial. (It would be nice to see a demographic breakdown of that 400,000.)
* House prices and the cost of living in the UK are insane, particularly alongside the heavy tax burden (an inevitable consequence of the enormous welfare state). I make two or three times the salary in the US than I would in the UK, and can actually afford to buy a home. This is not poverty per se, but there is a huge difference in how achievable are your life goals, and that's a motivation to move. As a young, then-unmarried man, there was nothing holding me back.
* The UK is an awful, freedom-crushing, oppressive, depressing police state where photographing national monuments is illegal, carrying a pocket knife can land you in jail, and defending your family against an intruder will net you a longer sentence than the burglar. I would never willingly move back.
Should I be granted US citizenship, I will be encouraging my brother to follow me off that sinking ship. Add two or three to that four hundred thousand.