Thanks for your question. I will try to help.
I’m afraid that you cannot bring her back with you on a visit visa for 6 months and at the end of it then switch to a fiancé visa. This is because a visit visa is subject to the condition that she must leave the UK before the expiry of it and this is taken to mean that she cannot switch in to another visa category (ie fiancé visa) from within the UK.
This means that your options are either for her to attempt to come back with you for a visit to the UK for up to 6 months, then leave before the end of it and apply for a fiancé visa back in Trinidad, OR for her to apply for a fiancé visa soon-ish (ie. without a further visit to the UK) from Trinidad.Either way, you best served by instructing a Uk based solicitor as soon as possible in order to commence work on the ultimate fiancé visa application.
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If she stayed for 6 months recently then the earliest time that she could enter would be January I'm afraid, because a visitor must not spend more than 6 months in any 12 months period in the UK..Tom
You would have to book the wedding prior to making the application because you need to show that you genuinely intend to marry, but if you wish to get her back here in the next 6 months then this would seem the most sensible way forward.
If you married in Trinidad, then you could apply for a spouse visa straight away provided you meet the eligibility criteria. The advantage with this is that once she is in the UK you would not have to make a further immigration application for a visa for another 2.5 years, whereas if you applied for a fiance visa you would have to make a further visa application within 6 months before her fiance visa expires and after you are married. (ie. to switch to a spouse visa)
The best thing you can do is to instruct a solicitor.