If my thai GF arrives in the UK on a tourist visa (6 months) and applies to extend that visa, therefore being allowed to stay in the UK for more than 6 months, will she then be allowed to switch her visa status to marriage/settlement visa.

Visit visas cannot be extended; except in extremely exceptional circumstances.

Someone in the UK as a visitor cannot apply for settlement in the UK; they have to return to their country of residence and apply there.

I take it this is a hypothetical question, so i will answer it hypotheticaly. No she can not extend her visit visa, Why do people try to circumnavigate the visa process? just abide by the the rules and all will be fine.