It's still for the DfT to decide on the AT300s. By no means a done deal yet.
Contingency, if no deal, is for 21 HST sets to remain for West of England services.
If the AT300s are ordered they may not have the same 'build and maintain' contract attached to them as the Class 800/801s, as they would be built in Japan, not Newton Aycliffe. Stoke Gifford in Bristol is unlikely to have the capacity to maintain another 29 (AT300, if ordered, would be 7x 9 car and 22x 5 car) sets. So its likely another depot site would be required. That would logically be on the line of route between London and Devon/Cornwall. Laira would seem like the obvious choice. Or a new build in the west. Either way there shouldn't be a dire threat to jobs.
I think the reports of Laira's demise are greatly exaggerated.
I'd be more concerned about Landore in Swansea...