I was browsing some penpal groups on Facebook yesterday when I came across this post:
I agree with the admin of the certain pen pal group and it is illegal to postally reuse postage stamps. A form of fraud or theft, although I haven't heard of any arrests/prosecutions for the crime.
Reading other penpal discussion groups and fora, this is a common practice. It isn't one I subscribe to myself. I buy my stamps from a lovely sub-postmaster and love being able to use the (mostly) wonderful stamp issues Royal Mail have produced to affix for postage.