Thanks Larry for the update on slit Machins. Here is a little more information which confirms that it will near impossible to remove these stamps from the envelopes once used.
(Thanks to John of Stamps of Great Britain)
Here is an extract from a quote from Royal Mail about the new Security Stamps due to be issued in February.
" The slits do not affect removal of the stamps from the backing sheet or from stamp books. An additional iridescent print working repeats the words 'Royal Mail' in a wave design in the background. The text is designed to be prominent only when viewed at an angle.
The new features will affect collectors of used stamps, as the new self-adhesive laminate does not allow stamps to be soaked from substrate. Whilst this is unfortunate for collectors of used stamps, it does reflect the need for Royal Mail to ensure that postal revenues are protected and costs are controlled. "
STAMPS of GREAT BRITAIN