I wouldn't worry about it too much to be honest. Even if TUPE, for any reason at all, doesn't come into operation, the effect of your losing the contract would be that the positions become redundant and the employees are dismissed. Redundancy payment qualification, as I'm sure you know, doesn't come into operation until after two years.
'Rolling' fixed term contracts are generally treated as continuous contracts of employment by tribunals who reason, usually correctly, that some employers use a fixed term contract to prevent workers gaining the protection of employee status.