Accomodation documents

Hello, i’ve been living with a flatmate for 3 years since i got to the uk and i am on a graduate visa. Only her name is on the tenancy agreement and the utility bills because we got the apartment before i got a job and we needed a stable income which she provided.
Although we’ve been splitting all the bills equally, i didnt have any problem with my name not being on it.

I am applying for a spousal visa under my partner’s visa, he moved in with me in april and we plan to get a place together before the end of the year.

My flatmate said we can just edit the tenancy agreement and add my name to it. She also said people live in house shares usually in the uk its not a problem as long as i have letters showing i live at the address.

Please What documents do i need to show ukvi to show validity especially as council tax bill is also in her name?