I'm not sure if I'm replying to everyone who has commented or a single response but thank you very much!
I'm a classic newbie.
That's really useful info! We haven't considered marrying officially in the US or even knew it was a viable option. I'll look into it, I'm actually travelling to see her next week and we'll be in Vegas for a week, home of the quick wedding. I'll do some research, but cancelling our UK wedding might be tricky as we've sent out all the invites, etc. We're actually having a blessing in Spain the following weekend from the UK wedding which we're treating as the real wedding for romance sake.
From all of the info I've read, we meet all the requirements. I meet the financial threshold, we'll live with my parents when she arrives as I don't want to choose a flat until she does (which I think is fine?) and we have a genuine relationship. Assuming a billion soppy pictures together, messages, holidays can prove that...