What documentation is needed for my future wife to travel to the UK aside from a visa letter and passport?

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Depends on which visa you are applying for. You need to provide evidence of your financial standing, that you have accommodation, eg mortgage statement, bank statement, salary slop, rental agreement. My invitation letter was shirt and perfunctory. She's my wife and it's none of the home offices business to know details of the relationship besides we are married. It worked. More complicated if a fiance I suppose. For a visit visa I was advised th suggest no more than a 4 week visit. I went for 6 weeks. You get a 6 month visa, but it's best to stick to your dates. My wife came to the UK in September and 6 weeks was enough to convince her living in the UK was not for her. Too cold and our food is no good. She has her own business in Thailand , and she knows the prospects of employment in the UK are very low.