I’d never much thought about this (like many things) but this is a promissory note that service personnel fighting in the first Gulf War were issued with. If anyone was shot down or otherwise caught in enemy territory, then the Foreign Office would give £15,000 to anyone who returned them to safety. They each have a serial number so that they could be verified.
Also known as a Safe Conduct Pass, they had been used in the past but as more of a demand that the member of service personnel was properly treated. This ultimately was more of a demand that may or may not be met, but the offer of money was a much more tempting arrangement.