Having more hours than I care to remember Paxing all over the place from LHR I agree with ETOPS on the issue that there is a huge amount of interline traffic at LHR. If there is less now than there was it is entirely because the capacity is limited in terms of runway slots and thus more flights.
Also while LHR has a very strong premium element the cachement area includes some of the countries most affluent areas and there is still huge demand for Y class travel. Firstly from the London area itself as none of the other airports offer anything remotely comparable for LH and all those BA card holders do have holidays and they along with all of west London N Surrey and East Berks want to fly Y from LHR and not travel 100 miles to STN however cheap Ryan air are. Equally Gatwick isn't exactly easy given that most of the journey is M25
LHR isn't ORD though because that place does have zillions of RJs and TPs serving a huge cachement area around Chicago and the upper mid west as well as being major hub and therefore it needs to separate traffic and run almost two airports inside one boundary hence all the runways.
So, LHR does need a third runway badly and building it at LGW would be almost criminal negligence on the part of the Govt. because it wont raise LGW status much and will ensure LHRs continued decline compared to AMS.CDG and FRA.
But then sadly decline is what we seem to do best in the UK with endless short term gain decisions either financial or political constantly winning out over long term strategy