My understanding is that airlines aren't really that interested in the ATPL ground school side of things as long as your not failing exams left right and centre, hence BGS will carry on just fine providing their excellent services. It stands to reason though that what airlines are looking for is a verifiable training record file comfirming your flying ability - i.e. one from a training provider that they trust because they have had a reasonable amount of safe training risk new entrants come through, which is unfortunate for some of the smaller training providers who often provide as good if not better training in some cases and still not have built up that relationship with the airlines. This way they can develop an understanding of what each of their favoured training providers condsider to be a good, average and poor student... So in conclusion, I think that 1 school is a good plan for at least a good chunk of your training, and if possible make it a reputable one. That's my understanding and experience anyway, not looking to invite an argument but reasonable opinions welcomed!