I'm a British Citizen, I recently went to New Orleans to get married to my girlfriend of 4.5 years. We're now in the process of applying/getting documents together for the UK Spouse Visa, but we're a little worried about my accommodation situation. I've been renting a room in a 5 bed house in London, however I've never had a tenancy agreement, or any form of contract confirming I live here. We (as a household), simply send a sum of money to the landlord once a month, and he let's us live in his house. My wife (then girlfriend), also lived here for 1.5 years before her study visa ran out, but again, she also never signed anything. The landlord has shown us proof that there is a deposit amount held by a third party, should he ever need us to leave, we can all get our deposits back.
What is our best way to prove that we have suitable accommodation for my wife to move into? Is a letter from the landlord going to be sufficient? or am I going to need to look into finally getting a tenancy agreement from him? We only plan on using this house to get her back in the country, afterwards we're going to look for our own place.
Any and all advice would be greatly appreciated

Thanks,
Rhys