The Predator Cities series title is a good move, I think, and this is a good time to make the change as the series will be re-published in the USA next month, too (with yet more new covers, which I'll post here shortly). If I'd set out to write more than one book when I started Mortal Engines I expect I would have come up with a series title back then, and perhaps it has been a handicap to the books that they didn't have one. Also, it was always my intention that the Fever Crumb books would be a separate series with a slightly different feel, so it makes sense to split them off from Predator Cities. Perhaps the Fever books will end up with a series title of their own one day.
(Of course, the chances of me actually remembering to call them Predator Cities after thinking of them as The Mortal Engines Quartet for ten years, are minimal.)