A question I have always asked myself is why a policeman doesn't pose as a customer and ask for x y z with the girl and gain evidence that way because it would be very easy to do. Or maybe that is illegal?
Under English Common Law, everything is 'legal' unless there is specific legislation to make something 'illegal'
As there is no legislation to make 'entrapment' illegal, in theory, the Police could do this.
There is case law that states that entrapment is not a defense, the Police do have to prove that criminal intent existed prior to the entrapment & that they did not lure somebody into committing a crime.
The Courts under Section 78 of PACE can exclude evidence that may have a negative on the fairness of a trial due to the circumstances in which the evidence was obtained.
In addition, to make a charge stick, the Police would not only have to solicit, they would also have to follow through & commit the offence with the SP.
So in reality, the Police can & will solicit an illegal drugs purchase for example, none will stand up in Court & testify to nailing a SP