This 'Stormont' label, as shown in the image posted by vayolene, was attached to a county definitive for Northern Ireland. These are Generic Smiler's Sheets sold at a premium over the cost of postage. Sheets were issued for England, Wales, Scotland, and Northern Ireland. Northern Ireland remains part of the United Kingdom (Great Britain is the big island). Northern Ireland country stamps are valid throughout Great Britain and Northern Ireland. They bear the cameo of H.M. Queen Elizabeth identifying them as 'British' stamps.
Quote:
and yet got cancelled as postage anyway?
No.
The label, very likely, would have been attached to the stamp when they were stuck on a cover. As a result, the label was cancelled together with the stamp.