Can i go to the orthodontist by myself without my parents? (UK)?!

Legally speaking if you are older than 16, then for things like medical procedures you are are a legally consenting adult. Your parents are no longer required to give consent for your treatment, its all now your decision, so the answer to you question is no, you do not need your parents with you

UK dentist

no dont think you need to... I was 16 when i first had mine.. and I only needed my parents on my first few appointments.. mostly to sign some documents etc.. but follow up appointments should be ok on your own. You can ask your onrthodontist if you are still not sure.

As you are currently undergoing treatment I do not think the orthodontist will expect your parent(s) to accompany you. A dentist wouldn't require it at your age.