I had a recent chat with a sorting officer manager (in fact, I've had quite a few chats with him). He told me the invisible franking showing up under a UV light theory is not true - there is no invisible franking.
He said that because stamp usage is so low these days, it is not cost-effective to have sorting office staff checking stamps on mail. However the postie should (if he notices) cancel a stamp before slotting the item through your letterbox.
He also said that only about 4% of post bears a stamp (obviously this figure will go up at Christmas) - most of the post is now franked, printed off on a computer, or printed by counter staff. I was quite surprised to hear that.