Boris Johnson has been out of office since September and you think thats who "wouldn't allow" something?
Since you put so much stock to what not-Ukranians say then lets have Biden explain it clearly to you:
United States is
“not going to tell [Ukraine] what they have to do.”
The United States is “not going to tell [Ukraine] what they have to do," the president said.
www.politico.com
We have our interests and we are an important ally to Ukraine, but at the end of the day it's their country and their call.
90% of Ukranians reject ceeding territories to invaders and there is zero reason to think Zelensky would want to.