UK Anc. Visa?

Hello,

i have a question about the UK ancestry visa, but it is rather complicated.

When i was a baby, my birth father left my mother and i and she later remarried.

Since my birth dad was long out of the picture, my last name was changed to the name of her new husband; I was never formally adopted, I just had my last name changed.

now, her new husband/my non-birth father since after the birth father left, his dad, my grandfather, was born in England. but since I was never officially adopted, and only had my last name changed to his, am i ineligible for this visa?

i didn't even know he wasn't my real dad until i was 25 so i don't have many details on this.

Any advise is greatly appreciated.