BA-FLY, I would imagine IAH would be the No.1 route to move to LHR because of extremely important transfer traffic to oil-rich states in Africa...as all of our sub-Saharan routes leave from LHR, that is where the IAH route should start/end.
I guess the lesiure routes (Florida, Caribbean) might stay at LGW as the company seems to reserve LHR for premium/business routes (that applies to Euro and WW).