What does it mean ¿A person who does an act which would constitute an infringement of the patent¿ under UK Patent Act?

There is some law suggesting that any acts -- regardless of the status of the actor -- committed after the priority date by a prior user are permitted as long as they are the same as the acts committed before the priority date. In theory, this would protect a distributor who was lawfully distributing the product before and after the priority date. However, these determinations are extremely fact-intensive, which means that you are not going to be able to obtain a definitive answer here. You'll need to review these matters with your own solicitor and disclose to him all of the material facts on which the prior use claim is based, including the facts relating to the status of the actual manufacturer. Good luck.