"Given there will certainly be a retained layer to manage the interface between BG and whichever, the actual 'permanent' job losses aren't exactly extravagant (if any)."
Bwahahhahaa. I work for one of British gas' competitors, and our idiot management outsourced the service desk to HP (along with a whole lot else). From a user point of view it has been an unmitigated disaster. I hate having anything to do with IT. I expect tickets raised to result in no meaningful resolution. I expect the Indian service desk to struggle to understand the nature of my call. I expect IT basics to not work reliably. Even simple stuff like synching and backing up user data from laptops simply doesn't work.
As for the job losses, yeah, everybody gets TUPE'd across. But then they are treated like shit, quickly come to top of the list in the outsourcer's next round of sackings, and the IT support is provided by more and more low paid, little experienced monkeys. Formerly stalwart IT expert colleagues are ground into the dust, to the point where they won't do a bloody thing without a valid ticket number.
However, this is British Gas, and they clearly haven't learnt from the mongo bust up they had with Accenture where they outsourced billing system development to that company and it all went shit shaped.