Almost all of the desired slots will be for an early morning arrival and a later departure. The additional slots that become available at present tend not to fall into this pattern.
And as Heathrow is also currently "full" for ground movements, stands, check-in desks and gates as well, there is that to consider. Can't quite see Continental going for transfer buses to/from the cargo area.
The whole thing about Heathrow access usually hides the facts. Heathrow is already full for movements. It's like a nice apartment building where all the units are taken by long-established residents. Here comes Mr Upstart, who now would also like an apartment. Sorry, it's full. All taken. But that's not good enough for him. Complain to the press. Say it's not fair. Whine Moan Pout Sulk. Throw the existing tenants out, make them double up, all so Mr U can get in where he wants.