Some constabularies get very excited about these. The launcher fits the definition under the Firearms Act. As you know these are 'one use only' and refills are not available (to all intents and purposes) therefore common sense should prevail and it should not be considered one. After all, there are numerous fully functioning (ex) Sec. 1 Firearms one can legally own without an FAC, as they are considered to be obsolete calibres but the ammunition is readily and legally available.
Safest course of action is to consult the local Firearms Licensing Officer.