Why LHR is so important as compared to LGW, someone asked?
Well, to start with the runways. LGW has one, which is inconvinient since when weather is bad, it slows down the whole airport, which means delayed flights, missed connections, which result in financial losses for airlines.
Also, since there is only one runway, this limits the capacity of the airport in terms of how many aircraft it can serve.
Also, the runway at LGW is shorter, which affects the weigth of the departing aircraft. The larger the runway, the heavier aircraft can go. You will note that there are lots of "heavies" at LHR, and they normally need longer runways to carry more people and cargo, so LGW runway is another penalty for airlines since they can carry less paying loads from LGW.
For airlines such as Virgin or BA, it is also cheaper to operate from LHR than from both airports since they have to shuttle all the time - pax, luggage. All of this affects connection time and missed flights, and also additional costs of transporting luggage and people from one airport to the other.
And since LGW is smaller, it means less aircraft to other destinations, which limits again its usability.
And also, LGW adds 20 pounds to the airfare of any LGW ticket, just to get to the airport from Victoria Station.
Personally, however, I like more LGW because it is newer than LHR, and is much better taken care of. Two weeks ago I was visiting London, and connecting at LHR and it was so dirty, overcrowded and very unpleasant, while LGW is more advanced with the electrical train cars that go to the terminals.
LGW is really very nice.
If I can think of any other reason why LHR is crucial, I will post it as well.
Hope this helps,