Over eighteen adoption and UK citizenship - is it possible?

I am 20 years old, and have been studying in the UK for the past 5 years with student visas. I am in university. My step dad, who has been in my life since I was young has been married to my mother for over 10 years. He is a British citizen. They live in the USA, but plan to move to the UK permanently in about five years. I am financially and wholly dependent on them.

Although I am over 18, my step dad wants to adopt me. This is possible and legal in my state. We will do this in the next few days.

If my step dad adopts me, will I gain any rights to UK citizenship through the adoption? Would I be eligible for an Ancestry visa, or any other UK rights? Because he has ultimately been my father most of my life, we are not doing it solely for visa purposes. Thank you for any assistance. I have done a lot of visa research on these areas but am having a difficult time pinning down a concrete answer.