Back in the days before the other London area airfields got busy I used to think that if LL 'sneezed' the rest of the LTMA 'caught a cold'!
It's quite telling how things have changed - highlighted greatly by the downturn in traffic.
The 'other' LTMA airfields have had reduced traffic but Heathrow has maintained and even had increased numbers.
The TMA has been much quieter which shows that when it comes to traffic, the 'other' airfields do indeed create a lot of work for TMA; in fact it is they who add the complexity.
However, if it does go wrong at Heathrow, it still has a huge impact on the rest of the room, just as I am sure it did in 'the old days'...