Given no unit has yet arrived in the Southwest (it's not even been officially confirmed that this is happening has it?!), and the units will require route clearing, traincrew will require training, as will maintenance staff (and given the units reputation, this is something that needs special attention), December definitely seems to me the earliest likely service date. That's also before you take into account that GWR may wish to make some of their own alterations (changing the livery, updating the PIS systems, restoring the driver door open controls?) before the units enter service.
Incidentally, the training course at TfW for drivers was 7 days long, and that only really scratched the surface. They're great trains to drive, but they do have rather a lot of unusual quirks.