Not for the first time the good Doctor Cable is talking out of an orifice most usually used for expelling solid waste.
Whether he and his LibDem chums like it or not, the vast majority of business people who travel by air are headed for, or departing from London and the Home Counties. As far as arriving tourists go, I would imagine that most of them are headed for a similar area. Adequate arrangements are already in place for UK tourists to depart from an airport close to their homes.
So there is no sense in asking a visiting businessman from, say, Hong Kong, to arrive at Leeds-Bradford or Exeter and then hop on a train which will probably increase his overall journey time by another 25-50%. The airlines know this (that's why you don't see too many departures from Leeds-Bradford to Hong Kong or Singapore) and so do the airport operators. Together they know a bit more about air travel habits than all of the government put together, let along "Vince".
Far from sucking the life out of the rest of the country London provides the life blood that keeps most of it afloat.