I'm with Telescopi on this. It seems a little dodgy and the Direct.gov website does say:
It seems unclear so I'd definitely have a quick word with a solicitor.
Ok slighly different scenario, you hire Amtrak to make regular delivery runs for your company, they turn out to be substandard, so you hire parcel force instead. You then have to TUPE Amtrak employees over to parcel force?