Hi and welcome to the forum.

I take it that both you and your girlfriend are Americans. I'm not sure what the UK stance is on issuing visas for unmarried partners, but if they do, she will not be able to work. If they don't, then she is going to have to qualify on her own for a visa, and that can be tricky.

Your lifestyle on £27,000 a year will depend on where you're located. Things would be tight in central London. Further out in the hinterlands, it could be quite reasonable, especially in a university setting where there are lots of facilities available to those associated with the university.
Cheers,
Bev