We know that CC has not covered themselves in glory but do we believe that they weren't aware of TUPE either. They have experience in awarding contracts and know the rules on TUPE. Look at it another way....
Did FK state that people were in scope as they were predominantly employed on those services? Don't know if they did but suggest it's doubtful that they said TUPE would apply. In any case, it's generally more an issue for a tenderer if TUPE is cited as a) you may have a different operating solution so don't need them or b) you could have people transferring on legacy contracts at a higher cost so would expect PCB would've been aware of this.
Obviously, in this instance, PCB would have been welcoming drivers, not least for the work that was transferring but also for the additional requirements that also needed to be catered for. So in this instance would PCB have had a reasonable expectation that they could recruit drivers in certain places where tenders were a greater proportion of the work and so drivers could be surplus? Well possibly but this probably where the issue is... Don't know if FK were running at a deficit with drivers but clearly, the management of AC have engendered a sense of loyalty, they elected to register a number of services commercially and they had a plan for other expansions so they could retain their drivers.
So whilst it wasn't unreasonable to think that some drivers could be available in places like Summercourt and Bodmin, how many GCB had bargained on..... well, you'd need to ask them. Clearly, they were trying to attract FK drivers (not surprisingly given timelines) by offering protected service etc. I don't think it was TUPE that caught them out but more underestimating what FK would do AND the loyalty of their staff.