GMB and Unison are extremely aware of how TUPE is worked under BSF and PFI as it follows some very basic outsourcing models as used by councils and other public sector areas for some time. The difficulty is where the unions do not ask for the contract or bid specific information from either the LA or the bidding contractor. Personally I would not rely purely on LA BSF Legal Advice as individual support, but rather as an access mechanism for information to your union. YMMV depending on your LA and what your LA has asked them to provide as support on the HR side of things.
I will also have to dig out the details from the PfS site but IIRC they strongly recommend that there should be no job loses in any bid they authorise to progress. Any re-org should be within agreement within the LEP and runs the risk of being regarded as in breach of the ability to deliver and would be refused. Remember that at contract point the MSP is expected to have already stated how they will deliver teh service and as such any change from that position is viewed legally and contractually with suspicion. Re-org with the intent of gaining more profit for the MSP is a big no-no, but re-org to ensure the MSP is financially viable is to be considered. If that is needed though, someone along the lines has buggered up the figures when working out what things will cost. At this point unions can (and will with pressure) get involved again.
However, an example might be a significant change in something like NI might affect the pot of money available for staffing (IIRC this impacted on BT in some of their contracts).