VIP LHR
From past experience, arrival procedure is usually done at the Heathrow Royal Suite near T4 or the Windsor Suite in T5. Same with departures (e.g. screening of pax). Signature used to have a lounge but closed it down and went into bed with the BAA to collectively manage the GA side at LHR.
DNATA starting at LHR I don't think will offer any competition. If anything, I think it will close Signature down and I don't think they would put up a fight. Signature might as well concentrate on redeveloping their LTN operation - with a new FBO and Hangar due for completion by Q4/2014, plus the heavy slot restrictions, congested airport and restricted parking at LHR...it seems like a winner to me!