are the people in Steria British nationals - can Steria guarantee this ?.
No, and no. Steria is a French company, but under the various idiotic free trade agreements that the British government have signed, there's little they can or will do to ensure that the work stays in the UK.
I know that they use offshore people a lot - as it is cheaper.
No, the oiks just get paid less. Lower factor productivity and the need of the vendor to recover the vast amounts of goodwill on their balance sheets (plus backloaded offers, aborted bid costs etc) mean that the savings of offshoring are illusory.
Why are no UK with UK based firms being asked to tender ?
It will have been publicly tendered. But anybody competent and with any sense of ethics wouldn't touch it, because they'll know that the desired savings can't be delivered for the client. Meanwhile, the buyer will listen to the snake oil salesmen explaining how much they can save by an efficient mixed sourcing model, and believe the lies. Why bid for a contract that you won't win? So long as the public sector rewards and encourages poor procurement practice, and fails to brutally punish failing suppliers, they guarantee that they'll always be taken for a ride.
how safe is the data that will be managed by the contract ?.
The outsourcer will promise whatever is required to secure the deal. But as UK newspapers found out some years ago, what a salesmen promises in the UK means nothing when you offer a few crisp notes to poorly paid offshore employees.