Factor 30:
Even back in the days before self-adhesive stamps (i.e. when you had to lick the back of them), I worked out what to do if I'd stuck a stamp onto an envelope where it wasn't required.
The answer is to simply cut around the stamp (leaving it attached to part of the envelope) and then to glue the whole thing onto a new envelope.
Dzug & Spudqueen:
The question appears to refer to the two oval 'cut-outs' which can be seen on each of the 'definitive' stamps here. (Strangely, Royal Mail have not included the same security feature on the two 'commemorative' stamps in the same booklet):
http://i39.tinypic.com/34eqm48.jpg
Chris