Questions: How long do you know her? Is your relationship secured?
If you really know her so well, love her that much then apply for Fiancee Visa and get married within 6mos and reapply again for another visa to extend her stay here. But you have to keep in mind about your finances coz she is not allowed to work in the UK for 6 mos.
Fiancee visa is much easier to apply than visitors visa.
God bless you.