Super safe?

Just as in the Netherlands, the British Post (Royal Mail) notices that a great many stamps on mail items reach the addressees uncancelled. Just as in the Netherlands, those addressees are happy with this gift from Royal Mail and reuse the stamps. 'Then they should just properly cancel the affixed stamps' is the defense against this 'criminal' offense. Is it so difficult to cancel all mail items to be sent? Apparently so. In the past, a security measure was already implemented to make photocopying stamps more difficult by creating an elliptical cutout in the perforation. A perforation can still be counterfeited, but it is somewhat harder with the elliptical cutout. However, the reuse of stamps that had not been cancelled was becoming increasingly common. Cancellation marks were washed away, making them appear unused. After much experimentation, Royal Mail was pleased with the following solution. A new stamp booklet was issued in April 2009 containing 12 stamps for second-class mail. But the stamps inside looked very different! Not so much at first glance, but if you look closely. The stamp is self-adhesive and, in addition to the old security features (elliptical cutout), has two elliptical slits. The British Post called the stamps 'tamper-proofIn other words, it could not be tampered with. For once the self-adhesive stamp from the booklet had been affixed to the postal item, it could only be removed leaving behind the two slits. In this way, the stamp was 'damaged' to such an extent that it could not be used a second time. And copying? They had come up with something extra for that as well. The stamp image was printed with faint letters spelling ROYAL MAIL in an irregular pattern. For the 'mint' collector, this is no problem: cut out the stamp and keep it as is. But what about the collector who only collects cancelled stamps? Are the stamps from which the two ellipses have disappeared still collectible? Where does the security arms race end?