It is irrelevant as to liability (fault) it is not irrelevant as to damages. Here is the relevant section of the UK's Copyright Act.
97 Provisions as to damages in infringement action.
(1)Where in an action for infringement of copyright it is shown that at the time of the infringement the defendant did not know, and had no reason to believe, that copyright subsisted in the work to which the action relates, the plaintiff is not entitled to damages against him, but without prejudice to any other remedy.
Seems simple enough doesn't it?