If you want affordable and a lot going on then you will probably have to come up further North for the time being. My suggestions would be Manchester, Nottingham, Leeds, Liverpool, Sheffield. Bristol would be great in my opinion if you do want to stay reasonably close to London. You could probably find somewhere decently priced.
Leicester could be good as well but never visited. Cardiff is also probably an alright choice but again never been.
Most of the places you listed are pretty dull and I think you will outgrow them quite quickly. I think you need to make this decision with your girlfriend though and actually visit places and look for jobs. Also could be worth considering renting for a year first before you actually buy. Just in case job situations change or you realise living in your chosen city isn't quite how you imagined. Although, from the cities I've suggested I don't think you'd have much issues with any them.