The UK fiance / fiancee visa is for those couples not yet married, but who want to marry and live together in the UK. It can only be applied for from abroad. It cannot be applied for from inside the UK.
Exceptionally during peak Covid, it was possible to apply from inside the UK, but this is no longer the case.
The fiance / fiancee visa is a 6-month visa, and the fiancé /fiancee visa requirements are the same as those for the spouse visa, except for one critical difference: You must show that you intend to marry in the UK within the 6-month validity of the visa.
The foreign partner is not permitted to work on a fiancee / fiance visa, but once married, they will need to apply for a spouse visa extension from inside the UK. This application is referred to as FLR(M). Once the foreign partner has switched to a spouse visa, work is permitted.
Beginning with the fiance / fiancee visa, the sequences of applications is as follows:
1. Fiancé / fiancée visa
2. Initial spouse visa (FLR-M)
3. Spouse visa extension (FLR-M)
4. Indefinite Leave to remain (ILR)
5. British citizenship
So, the fiance / fiancee visa is essentially an additional step, added on at the beginning of the spouse visa route. For that reason, it is a more costly route. If you are able to marry outside the UK, and apply straight for the spouse visa, this would be a cheaper option.
You can find a more detailed explanation of the differences and considerations when choosing between the spouse, fiancé, unmarried partner, and marriage-visit visas, here: UK partner visa routes - What's the difference between them? And also, here: What's the difference between a fiance / fiancee visa and a spouse visa?
What are the fiance / fiancee visa requirements?
The financial requirement is the most complex part of the fiancee / fiance visa application, and also the most important. It can be met in various different ways, so it's difficult to summarise in a just a few bullet points. But by way of an overview:
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