I agree that you should first find out if it's your baby, but I wouldn't hold out much hope. Yes, she lied to you, but "I changed my mind about the abortion" is a very different kind of lie than "I had sex with somebody else around the same time."
As the father, you have certain responsibilities. You might even be legally obligated to pay support -- I don't know how things work in the UK. You should see a lawyer, or at least do some research on the Web.
As for moving in with her, I'd think very carefully about that. Unless you're planning to live as a couple, any other arrangement will probably drive you both crazy within a few weeks, especially with a kid needing constant attention.
Perhaps a better solution is to act essentially like a divorced dad. Live in the same town so that you can see them as often as possible, but don't move in with her.
And in any event, I'm thinking the best thing that could happen for all of you is for her to find some other guy who will love her and can be a stepdad for the kid. Since you said you're not willing to marry her, at least leave her free so she can find somebody who will.