From the Legal Aid website: “Your landlord can no longer give a ‘no grounds’ termination at the end of a fixed term agreement.”
So you can’t tell them to vacate at the end of a fixed term unless you meet one of the fairly-well published criteria (you are selling, you are moving in etc etc, I’m sure you’ve seen the list).
You telling them to vacate at the end of the fixed term is considered an “eviction” if they don’t want to go. Refusal to sign a new fixed term is not a valid reason for that eviction.