[Help Request] Proof of Residency for Registrar

My Nationality: British, born.
My Partner's Nationality: United States, born.

Hello there! My partner and I were recently accepted for a marriage visa. They're flying in soon and we've been talking to the relevant parties, though an interesting wrinkle has come up and I'd like to seek some advice from those with experience.

When we meet with the registrar to arrange our marriage, we'll need proof of residency. First of all, they told us that we should get that from the housing association that owns my house. I tried that but I had no luck, an irksome Catch-22 of requiring that my partner having been present for twelve months and married already before they'd even consider it.

We reported this back to the registrar and they gave us advice that, to us, seems a little sketchy? A touch shady?

They asked if we had any home owners in the family. I have a sister who owns her own house. They said that we could have her write a letter claiming that my partner is living there. This is problematic. That's not the house that was agreed upon in the visa. Photographs of my home were sent, along with a statement of suitability regarding my home, along with my tenancy agreement. This was advised by a lawyer and valid—of course—as my partner has already been granted a visa.

I don't really know what to do. If photographs, written letters, housing tenancy, and such aren't enough for the registrar then should I go along with their plan? I'm really scared to do so, honestly, because if home office compares records and those records say my partner is staying at two different places then that could be grounds to have her deported.

I'll be speaking with my lawyer on Monday for further advice as well, but I just wanted to see if anyone had experienced this and how they solved this particular pickle.

Thank you so much for your time and any advice you can provide. I really, really, REALLY appreciate it. Really, thank you!

Put your partners name on one of the bills/utilities etc.