LHR is full. Have you:
- sat in the taxy queue for departure for 45 minutes? (engines running)
- been onboard when a slot has been missed?
- arrived on time and then been in the stack for 20 minutes?
- arrived at the allocated gate but it's still full and there is no other gate to send you to?
- waited for immigation?
- waited for bag handling?
These are but a few ways that LHR has dealt with the problem. If they had to prevent all routine inbound holding and outbound taxy delays? Then count how many a/c they can handle.
Bear in mind that, in some ways, LHR is rather like the NHS. The NHS broke ten years ago but is kept going by staff working above and beyond and without overtime. LHR is kept working because they have bent all the rules and restrictions and enough clients STILL want them enough to pay through the nose. Both LHR and the NHS are providing a service long after most others would have ground to a halt.
LGW (and I'm not anti) has benefitted by having flights taken away from it that has allowed more new clients to come in and keep it busy.