Well, like someone else said, while slots at LHR are few and far in between, given enough incentives, some may be found. The EU will be looking askew at britain, if the big hold up keeps being access to LHR.
Secondly, while it is true, that slots are "owed" to foreign carriers, it might just be more important to give them to US companies as opposed to "Kablakistan Airways".
Of course, the british goverment and perhaps more importantly, BA and to a lesser degree VS, is very much against opening LHR. Too much of a gold mine.
So, I guess they want to make an omelette without breaking the golden egg.