Hmm I wonder why.....maybe everything goes through London as...
1. Its the Capital of the UK
2. Its the Business Heart of the UK
3. Its where all the Major Businesses are based
4. The South Coast has the UK's Silicon Valley (London to Newbury to Guildford and everything in between.
5. Its the capital and heart of everything as its closest to Europe
Yes Manchester, Edinburgh, Newcastle, etc, are all big cities but frankly none of them are as vital to the economy as London. The same argument can be said of any nations capital.
Paris......as she loves mens cables, and deep dark holes ;)