The 1999 CAMRA survey showed that 91% pubs served short-measure for pints.
Trading Standards experiences across the country have varied as to whether the 95% tolerance level is met. Pints glasses should have at least 95% of a pint as liquid - this is a Brewers and Licensed Retailers Association guideline.
In 2000 CAMRA surveyed with a new tool: a chemical that collapses any beer head to allow accurate measurement of the liquid. The findings were that on average:
The real ale newsgroup around September carried a discussion about the common practice in big pub chains to leave pump clips on display after the barrel has finished and other forms of offering non-existent beer.
Cambridge CAMRA activists have noticed the two Hogsheads here have been particularly bad in this respect and so we mentioned this to Cambs. Trading Standards. Their reply came back just before Christmas: their inspectors have observed the same but they feel there is no current law to be enforced in this area.
In several key uk.food+drink.real-ale articles Alex Koval described several relevant cases. He reckons that handpumps which falsely offer unavailable beer are covered by the Trade Description Act 1968, Section 1(1)(a), and this apparently was used against the Wetherspoons in Bury (Lancs.). In this case they advertised Spitfire for weeks when none had arrived. However this never came to court.
Whilst it seems many local Trading Standards do not see a way to proceed, he explains there was a successful prosecution over five years ago by Tower Hamlet Trading Standards under the Food Safety Act 1990 of a pub which supplied something other than Fullers London Pride upon being asked for that (which hadn't been stocked for some time).
The upshot seems to be that merely having misleading pump clips on display is not an offence but that more extreme forms of offering non-existent beer may be.
A key point is whether a misleading clip represents an invitation to treat - an indication that the product is on offer.
All of the following (and probably others) can be relevant - it seems this area of the law is a mess!
Branch Trading Standards page.