I would say that BAA and ATC have been sucked into it as its bad press not to go along with virgins idea and at least try it,
On the first test at heathrow, it was due to be a 1000 departure, it then got pushed back to a 1500 departure i wonder why,??
The next thing is request for engineers and a air start out to the hold no doubt.
They would need more staff, more tugs, more time, and no doubt on busy days aircraft taxiing behind the towing aircraft and would use more fuel??.
Best to give it a go i guess it may work,