You should consult with the embassy in the UK to get answers to your questions. However, I believe your girlfriend is old enough to legally get married. I am assuming you are also of legal age. In the meantime, I have posted a link to the EasyExpat guide to London where you can find general information on visas, etc. Good luck. http://www.easyexpat.com/en/guides/united-kingdom/london.htm

 

Hey there,

Take a look at the UK visa bureau website (http://www.visabureau.com/uk/marriage-visa.aspx). I got this info from there:

Applicants who have been in the marital relationship for less than 4 years will be given a 2 year visa to live and work in the UK, after which, if the relationship is still in place, they could apply for Indefinite Leave to Remain (ILR).
Applicants who have been married and lived together for 4 years or more with their sponsor outside the UK before applying to enter on a marriage visa, may be granted Indefinite Leave to Remain straight away.

They also have a free online visa assessment. If you've got the money you could also get a visa representative.