In the UK you don’t need a license for a blank firing weapon, you can actually buy the blanks off Amazon with no proof of age etc. You DO need to be over 16 years old to own one.
You need to be a member of a group or organisation too.
As for deacs, the certificate is just a piece of paper. You don’t actually need a certificate for a deac. You do if selling it on tho.
As long as the weapon is proof marked on all the parts deactivated, bolt, barrel etc, it is legal. Some displays will insist on documentation also to attend.
You need a certificate if taking your prized welded up paperweight to Europe tho. To prove the deactivation is to their current butchery laws!
Some countries, it’s actually easier to own a live firing weapon such as Bren, Sten, Enfield etc than a deac!!