Upvote:7
There is no route to extend a visitor visa beyond the standard 6 months unless the visa holder is:
a patient receiving medical treatment
an academic and you still meet the eligibility requirements
a graduate doing a clinical attachment or retaking the Professional and Linguistic Assessment Board (PLAB) test
Source: https://www.gov.uk/standard-visitor-visa/extend-your-stay
Even giving notice of intention to marry https://www.gov.uk/marriages-civil-partnerships, and assuming the Home Office approved the request, would not mean that your girlfriendβs visit visa would be extended beyond the original 6 months.