Unless the UK is very different, it shouldn't be entrapment to let them try to buy it. IANAL, but in the US, entrapment as a defense requires "the defendant's lack of predisposition to engage in the criminal conduct".
It's relatively readable, it's probably enough to note that a "stay" means that a trial will not proceed (at all).
A quote from that section:
"Police conduct which brings about state-created crime is unacceptable and improper, and to prosecute in such circumstances would be an affront to the public conscience. However, if the accused already had the intent to commit a crime of the same or a similar kind, and the police did no more than give him the opportunity to fulfil his existing intent, that is unobjectionable."