If copyright subsists in the sweater, then it is an infringement to copy the sweater (again, this is subject to the doubtful proposition that such a replication might, just might, fit within the scope of fair dealing). However, if the sweater falls into the industrial design exception, then it may not be copyright infringement (although it might be industrial design infringement, if registered). Remember, this industrial design exception is relatively new and untried.
Even if you think that the industrial design exception applies, you must be sure that more than fifty copies of the sweater were produced. And then, there is the question of whether there is industrial design infringement.