I haven't heard of any specific case ruling on these - you hear that some police forces regard both cups and spigots being considered prohibited weapons, whereas others see them as Sect1, and yet others as un-licensable militaria. Its one of those many firearms areas where someone has to be the first to risk 5 years in jail to find out judicial opinion!
I think that, if police wanted to be unpleasant, they can get you on the "designed for" bit. Cups and Energa spigots are quite clearly "designed for" launching explosive munitions, albeit it must be quite a fine dividing line given that AFV smoke dischargers are not regarded as firearms....