Mickyman - this is the estimated origin / destination of LHR terminating passengers. It is a measure of where people start or end their journey, not where they live, so obviously overseas residents landing at LHR and then commuting onwards will be included in these numbers.
Connecting passengers are not shown, but should be easy enough to work out from the route data the CAA publish monthly as is freely available.
Clearly if the government are looking at proposals to take regional traffic out of LHR, the South West and the Midlands are the biggest opportunities
1,353k East Midlands
3,828k East of England
83k North East
427k North West
89k Scotland
29,736k South East
2,835k South West
746k Wales
1,186k West Midlands
570k Yorkshire & Humberside