AtlanticFlyer
Virgin (aside from potential BMI purchases) can only wait until T5 comes online and the respective terminal (& stand) capacity increase allows additional slots at better times to be released.
Whilst I wish it were as simple as this, the main problem at LHR is runway capacity. We are at around 95-98% capacity at the moment with the slack being at times that aren't profitable to fly. The quickest way for Virgin to expand would be to buy BMI and kill off the poor routes so they can use the free slots to expand their long haul network, whilst using the profitable short haul routes to feed onto the long haul network.
Or they could follow BA's strategy, to pay airlines to move out to Gatwick and then buy their runway slots, which they've done with quite a few european short haul carriers, most recently Malev.
Indeed, this is the only way BA are able to expand their number of flights in/out of heathrow.
Whilst stand availability is an issue it's nowhere near as important or critical as runway slots.