This is undoubtedly true in the UK or any other country where feral ferrets exist and interbreeding has occurred. In areas where there are no feral ferrets, there are clear cut differences between polecats and ferrets, such as skull features, orbital angles, body structure and muscling. Most of all, the differences in speed and agility between a polecat and a hybrid are striking, even more so between a polecat and a ferret.
It's possible that pure European polecats exist in the UK but, given all the photos I've seen of supposed polecats, they must be extremely rare.