Thanks for your question.
It's not good news I'm afraid.
She cannot apply to switch in to any other category of visa whilst here on a 6 month visitor's visa. It is a term of such visas that the visitor's must leave the UK before the expiry of the visa and this is taken to mean that they cannot switch in to another category (including settlement).
If she does not leave before the expiry of the visa then it would put the UKBA on a very adversarial footing for any other visa she would apply for and this could cause serious problems for you.
My advice if you intend to settle here permanently would be to go and see a UK immigration solicitor in your area two months before the expiry of the visa and ask them to prepare her settlement application for her. This way she can take it back to Thailand and submit it straightaway.
If the application is well drafted, appropriately evidenced and clearly laid out then the chances are that it will be approved quickly.
55% of applications from Thailand are determined within 15 days of submission. Having a solicitor prepare the application should hopefully ensure that she is in this category.
Sorry it could not be better news.
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