Here's a paragraph that might explain the decal:
"An example of a territory which was not party to the treaties permitting free circulation of foreign vehicles, was the TANGIERS international enclave, and to visit Britain after the WW2, this owner was given QC (foreign visitor to Britain) plates to permit his entry. Q-C was allocated to, and handed out by, the the Royal Automobile Association (RAC), as were Q-D and Q-H. The RAC and AA auto clubs assisted travellers with the considerable documentation required for international travel in those times – Motor Insurance, Carnets (partly to prevent the sale of cars in foreign countries), supply of International Ovals (seldom available in the originating countries) and so on." https://blog.europlate.org.uk/category/tangiers-free-zone/
The decal in the picture shows it to be a RAC product so my guess it wasn't something issued to visitors to britain to get registration plates to permit driving around. Perhaps an inquiry to the RAC might be of help. I know that owners of RAC badges have had good results contacting them for information regarding their individual badges.