I once deliberately put two in the hold at the same level. They were at opposite ends of the racetrack, speed controlled, and never any chance of colliding. This was, of course, in the simulator during one TRUCE session, I would never want to do it in real world.
The LAM, BIG, OCK, and BNN, holds give the area bods a fighting chance of dealing with non-EGLL traffic, and not whacking the planes that matter to you. Folks know where the a/c are, they're not bimbling about in the most congested/complex bit of Europe's airspace.
I totally agree. Having that relatively small area of known holding airspace makes it a lot easier to get other aircraft past them.