no one else in the world could provide the combination of tech and services NEST requires
Seriously, they expect us to believe that literally nobody else in the entire world is capable of building a pension administration system? That the challenge is so great that these wizards of mundane data processing are the only choice?
The excuse that they've made big changes to the specification, but refuse to pay for the extra work or change the delivery deadline, so they're changing the supplier because the contracted supplier refused to do this looks very questionable from a contract law point of view. Normally this would lead to a court case, because nobody would sign a contract with open ended free future work requirements. My guess is that ATOS has too many other contracts with the government to make them dare take action.
There's also a suspicion that this is what a "tilt to the indo-pacific" looks like in reality. Never mind, when you discover just how high the quality of work these Indian consultancies produce is, and how they never bilk you out of more money at every opportunity, you won't regret this move.