It seems to me the logical thing to do could be that you should get married soon and then for her to apply to the British Embassy for a "spouse visa"to go over to live with you in the UK.
That could take some time, if it's anything like it was for my Peruvian wife in 2006 through the Australian Embassy system. There are quite a few requirements, so the sooner you and her investigate that aspect, the sooner you can achieve your wishes.
Oh, after 11 years in Aussie, we have now returned to Peru to live...it's my turn away from my country.
Hope that helps a bit.
Retired Aussie in Lima.