Surely Servisair (as an example) could manage bar trollies at LHR as well? If BA keep on with GG as their only supplier, they're going to land in the wotsit again - and again - and again. Every time.
They have got themselves in a situation where they have an unmanageable risk as things stand, and they need to get out of it. I feel sorry for all the staff that are effectively 'at risk' through no fault of their own because management at BA can't (or won't) do something about a situation where they can be (and were) blackmailed.