It's only a trial to test the viability of the project and analyse the savings etc.
The handling agents don't at present have the resources for it to be launched immediately but as I said there is no need for it at present and no ground power is required as the aicraft have an APU. In the event of an inop APU then an on stand engine start would have to be carried out as normal.
I should point out the trial isn't just a Virgin project it is being run along with BAA and NATS.