London is a Primate-city. it is home to 13% of the entire population of the country. There's nothing really comparable to that. It's the financial, business, economic, and social center of the country, if you took away that city and looked at only other cities, the country would be a shell of itself.
Greater London's population stands at nearly 10 million, almost four times that of UK's second largest city—Greater Manchester. On the other hand, Greater NYC has 20 million, while the second largest city, Los Angeles, has a close 18 million.
Why would you even want to go anywhere else? Who wants to buy a house in a town in rural Britain or live in a place called Bristol? Sounds mad depressing.