Sorry but they aren't.* If asked they must tell the truth otherwise it is entrapment.
* Unless policy has changed, that was certainly the case when I did some work for trading standards some years back. I still can't see a defence losing if it can be verified that age was requested. I don't believe it is law to ask for ID, just policy.
Which incidentally I don't agree with. I don't drive, have no intention of doing so, refuse to carry a passport around, or "buy" an ID card. If an establishment refuses to serve me (not that there's much chance of that in Croydon) I will simply go elsewhere. I'm 33 and look it and have never actually been asked for ID in my life anyway.
The law was changed a couple of years back so that the test purchase could be more realistic.