Hello. A few days ago I showed an image of a a stamp label ( similar to the one above - another I found in some old stock albums I somehow accumulated)   Thanks to Larry Rosenblum  for the following :

Continuing a practice started in Victorian times, until October 31, 1973 the British Post Office would impress stamps on private stationery and self-adhesive labels. The indicia (markings on a mail piece showing that postage has been paid by the sender)  were those used on official postal stationery. The same impressions were used for this stamp show sheet. It was sold for 30p and 120,000 were issued. My copy of British Stamp Exhibitions from a few years ago values it at £2.50, and I suspect it is not much different today. 

and, if you are not reading his Blog I highly recommend it .. at  http://machinmania.blogspot.com.au/   brilliant blog.. it is in my list of favourites of course ..Machin Mania.

Enjoy your stamps...  Michael