I may not be the best one to offer advice. I'm a 50 year old man. But I do have some life experience and a daughter a few years younger than you, so I can visualise your situation as if it applied to her.
I would not consider marriage, or uprooting your whole life. You say you have a career, a good network of support, etc. where you are. You will be a better parent if you are happy and supported, even as a single parent (and there's not much stigma to that nowadays - in many places (including parts of the UK) it's almost the norm).
A loveless marriage may well bring you down, with the resulting effect on your child. There is a balance between making a sacrifice for your child and doing what's best for you as a person.
You say it's been a 'no commitments' relationship, and pregnancy doesn't automatically change that. He has commitments in terms of child support and obviously some sort of 'access' rights, but that is, IMHO, no basis for a marriage.
If you married him and moved to UK, there is a distinct possibilty of legal complications and restrictions (e.g. if you separated you may have to get court permission to take the child out of the jurisdiction of the England and Wales courts). You would find life in the UK very different to life in Nevada (not necessarily better or worse, just different) and dealing with that could be difficult enough without having to look after a (first) new baby.
Try closing your eyes and visualising youself, your baby and your life in 5, 10, 15 years time. Do you see your future with this man? (I suspect you already know the answer to this.)
You MUST consider your own happiness, because that will reflect on your relationship with your child.
Good luck and best wishes whatever you decide.