In any case the law is intended to prevent dickheadery on a monumental scale, not to stop us adding a much needed dimension to airsoft, accurate indirect fire, or to stop people having a laugh with well designed safe equipment.
I'm hoping that these fall outside what the law defines as 'explosive' for the purposes of the law and perhaps someone can enlighten us on that score. Indeed there is some question over their legality under the letter of British law, since here it definitely is illegal to fire explosive projectiles out of/with anything.
They're not new, but until recently were not available from UK retailers.