To my knowledge Mark - no licence is required but boy, do you have to be careful.
Check with the show organisers if it's OK as people tend to get nervous when firearms start to appear.
Always advisable to check things like liability insurance if it's a public event.
Advisable to inform the local Police as well or you may well end up with SO19 pointing real guns at you.
PLUS.
Never, never, ever point a loaded weapon - even with blanks at anyone or anything. Some people presume a blank is harmless - not so - seen many serious injuries with blanks. They do fire projectiles and can give a serious burn.
Not saying you would and don't want to teach people to suck eggs - but some people are very blase about blank weapons.
Markheliops