why is there not another less busy airport to land at in such a situation
So Gatwick and Heathrow are both RFF (fire fighting) category A10, which dictates the number of appliances and staff immediately available! And based on aircraft length and width
A 747 requires A9, but can in some circumstances accept A7, of course this is an aircraft with a serious issue so will require a full capability
Stansted & Luton are A7
So where would the quieter airport get the capability to deal with a larger aircraft if they don't have it? They can't take it from one close by as that would reduce that one's capability. Or should a random location be A10 and have equipment and staff doing nothing but wait for a divert! Who would pay for the rare times it would be needed?
(A10 BTW is for an A380)
Airport Categorisation