Immigration Law Questions? Ask an Immigration Lawyer.
Thank you for your reply.
If one of you is a US citizen, you are taking a chance that USCIS will deny admission IF YOU TELL THEM that you will be getting married when you enter. If not, you can marry here, she can return to the UK and then you can apply for her to come to the US as your spouse through the US embassy in the UK
You cannot speed up the government I am afraid. 18 months is usually the outside maximum length of time, it could be approved in 6-10 months, but none of us dealing with these cases ever know how quick it will be because it depends on the USCIS agent who gets the case and availability of documents.