There is an interesting article in the Economist that argues the real answer is to let the 30% of passengers who transit Heathrow (rather than originate or complete their journey there) go elsewhere (FRA, AMS, CDG). The UK does not derive any real economic benefit from transit passengers, although BA and BAA might.
The 'Heathrow needs a third runway or the City of London dies' argument collapses if you take out transit passengers.
The logical place for Heathrows third runway is the Continent.
Ok, so if you take out the transit passengers and let them go elsewhere then the direct flights then become unviable, so some of those too would go.
Of course the UK gets economic benefit on from them, a lot of them travel on UK airlines which if I am not mistaken pay money to the UK and benefit the economy. Take the transit passengers out and then the economic benefit will go elsewhere and not the UK!!
If the passengers went elsewhere so would some of BA's operation, meaning job loses in the UK! Maybe Open Skies is going to do some of that, but that is another debate!