IMO, Aviance are about the best of the contracted agents at LHR, they handle some pretty demanding airlines like SQ and do it very well with good service on check in from pleasant and cheerful staff (with the very odd exception!), efficient boarding and good baggage delivery on return.
Compare them to Globeground and Aviance shine very brightly with the latter being represented by some of the surliest, rudest and most miserable people on the planet (maybe that's specially for gold card holders and they are worse with others) and baggage delivery that is unspeakably slow (and LH crews wonder why their passengers take too much hand luggage on board!). Try comparing Globeground handling LH in T2 and AZ either handling their own services or LX and AZ are the winners by a very long way - maybe not what you would expect but my experience time and time again and the AZ staff are pleasant.
I just hope that when *A move in to T1 that if there is to be a single contracted handling agent then it is Aviance ahead of any others. There's something seriously wrong with GG where the culture appears to be to treat all passengers with as much contempt as the staff member can muster on all occasions.