JSP 761 is the gospel on such matters.
https://assets.publishing.service.go...P761_Part1.pdf
It comes under the command and direction of the Defence Council (heads of the Military, Ministerial and Civil Service pillars) who direct under the scrutiny of the Monarch (every new award recommendation will be presented to the Palace for approval). So, if the JSP says “no” then you shall not wear it.
Of course as a civvy you can become a “Walten Commando” and wear what you like. You can buy all of the unofficial bling from the likes of Award Medals (
https://www.awardmedals.com) and dress up like Uncle Albert from Only Fools if you like - sadly there is no anti-Walt legislation in the UK like other countries. The only defences are the newspapers and the Walter Mitty Hunters Club:
https://thewaltermittyhuntersclubhq.com who will publicly call out any Waltish medal groups worn at memorial services.