You don't need to be married or a citizen to have proof of mail - people applying for the Unmarried Partner visa have to show the same mail evidence to prove they live together... because they are NOT married.
Ideally, as soon as she arrived in the UK, you should have added her name to as many bills and other official mail as possible, then saved everything that came in the mail.
It doesn't matter what address the mail is from - what matters is that you were both living at the same address as each other... you could move 10 times and you would just provide mail from whatever address you were living at during each 4-month period.
If you don't have any mail in her name yet, then it's not an issue... just include a letter explaining why you haven't got any mail in her name yet.
For the record though, you will need to show this same proof of mail for every visa in the next 5 years, so to prepare for it, as soon as you can, start getting mail in both her name and your name (either joint or separate).
So you can do things like:
- get your names on some bills if you can
- get paper statements for everything
- open a bank account for her if she hasn't got one already (though this will require proof of address)
- register with the NHS if she hasn't already (this will require proof of address too)
- after the wedding, apply for an NI number for her
- she can apply for a UK provisional licence
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