Disclaimer: I have never worked for VS, or met Sir Richard, just a satisfied customer of 25 years.
Firstly, it's his job to point out the deficiencies of the system. He can get publicity for the problem of constraint at LHR that his CEO cannot. Indeed that very few can get and that certainly includes Mr Walsh.
Secondly, after Sir Michael (BD) turned them down, they have not been seriously interested in a domestic network as they have done very well without the problems of having one. Certainly who could see them really wanting to start lots of domestic connections on minimum margins? Further, in the past 20 years, the UK has seen the rise of the LCC and regional operations by a number of international carriers. If they could have bagged a domestic carrier 10 years ago? Perhaps. But not now, I think.
Thirdly, if they took the slots back - who is to say that they are at times of day and days of week that would suit their kind of operation? They have very specific kinds of routes which those slots might not be suitable for. Further, in the middle of the darkest recession, would you want to be buying/leasing new aircraft and expanding the operation - or quietly taking the slot money and making sure that you keep the whole airline stable, whilst waiting for better times?