Timey-wimey
Harold Saxon and a strange anti-Doctor conspiracy had already appeared in present-day Britain by the time we saw Derek Jacobi. Regenerating into John Simm was the surprise reveal.
In a behind-the-scenes programme, Derek Jacobi said, without mentioning alternative commitments, that he'd have relished spending more time being evil as The Master. But the plan was to Simm him.
And it wouldn't be a surprise if we could see it was him all along. I suppose he could have teamed up with the Slitheen this time and got a human skin costume of his own - but the Slitheen had already had one go (actually successful, briefly) at taking over the British government. And there's, er, the gas problem.