Although we live close to London City our kitchen and back garden are visual with the LAM holding traffic, so I get a good idea in good vis of how things are going when I am doing the washing up !
One thing noticed during the BA strike last August was absolutely no holding at all !
When inbound from the North East, which as pax I am quite often, the first sign of trouble is if you are held overhead the Clacton VOR. This seems to be done when LAM is getting up to capacity, and has always in my experience been an indication that you will get a LAM hold as well, so the question of which could be the busiest is a bit complex as part of the load is held further out.
Lambourne VOR is on Stapleford airfield (home of G-WHBM) and in good weather out there it's always interesting to note the quantity of traffic holding.