I haven't had any contact with my mum for at least 7 years (apart from seeing her at my grandma's funeral). I moved out of her home at the age of 16 due to a number of reasons, including her addiction and domestic violence issues. I couldn't move in with my dad (due to his fear of past DV issues), so my sister took me in for a while until he was ready.
My dad has a fiance of at least 15 years and she is amazing. I consider her my mum but call her by her name. I struggle to call her mum from sheer awkwardness (my own) I think - although I do get her cards with mum on for occasions etc.
I really love her and although I think she knows this, I'd love for it to be official that she's stepped up when my own mum was shit. Now that I have my own child, I really appreciate how she has been there for me all the more.
But after looking into it, adoption only covers minors in the UK. And I'm really disappointed.
I understand that you don't need a piece of paper to show that someone is there for you but at the same time, if it would just be a piece of paper for someone over 18, why not offer that service?