Can my step dad adopt me if him and my mum have separated

Just for the record, I am 18 so this is different than an underage one.

I want my stepdad to adopt me, I don’t have my birth dad on my birth certificate and I don’t know who is he is in general. The step dad in question was my step dad from when I was 5 up until recently of last year, so he is the only dad I’ve ever known and I really want to have him adopt me but he has his own house now and he still acts like he is my dad completely and him and my mother have had my 3 siblings together.

I don’t know if all this context is helpful but I just want to know if he can adopt me and how would I go about doing that, I’m from the UK!