surname change (legal age)

I'm 18 years old turning 19 this year and I would like to legally change my surname into my step father's surname (I am not adopted but my mum and step father is married in UK). I m currently using my biological father surname, my mum and biological father is legally separated by law and I did not receive any support from him since I was a kid he actually abandoned me and my 2 siblings (both in legal age). Also my surname(biological father surname)was always mispronounced by people and it's humiliating for me, something that I was never proud of. Can I change my surname legally and use my step dad's surname even tho I am not adopted? My step dad support us in the very first place. I hope you can help. I don't wanna graduate senior high and college using my biological father surname I wanna use and be known with my step dad's surname