Legislation requires that an imitation firearm has to be realistic to a real firearm. Airsoft and pyrotechnics grenade launchers are not firearms, nor are they an imitation firearm as generally they do not look like a real gun.
Some launchers come with a shoulder stock (STA / Stand Alone) and pistol grip and may be interpreted as an imitation firearm, however the courts have not been asked to clarify these types of launcher.
If you own such a launcher then we advise that you do NOT carry it in a public place as the police may treat it as an imitation firearm as described above.