Yes, Heathrow is very congested, which is why a lot of airlines have operated into Gatwick due to problems obtaining slots of Heathrow to access the London market.
Once airlines they have built a presence at Gatwick their thoughts often turn to Heathrow access and it usually takes several years of negotiation and bargaining with Airport Coordination and other airlines to get the slots to count a viable service from Heathrow.
The airline airlines moving from Gatwick to Heathrow listed above have done so over the last decade, so there aren't exactly a huge of services being transferred on an annual basis. Air Seychelles only required 4 LHR slots per week to move their flights, and these are all in off-peak periods.
There is no line that can be drawn regarding transfer of flights from LGW to LHR. If an airline can obtain suitable Heathrow slots and the bilateral air service agreement involved allows it, then that airline is free to transfer services from LGW to LHR. Similarly the reverse is true, Heathrow slots are very valuble, there was talk in April of British Airways buying LHR slots from Lithuanian Airlines, Balkan Airlines, Adria and Avianca in exchange for Gatwick slots and cash to allow expansion of their main hub.
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