There was a case in the 80s when a well know wg was taken to court, can't remember her name though. HMRC is a joke. I've not worked self employed for about 15 years now but I still get letters telling me I'm being penalised for late declaration.
Most grey economy elements are consistent in prices, not much variation in the price of drugs nationally, but the variation for wg's is huge, £5 crack BJ to high end over night. Would make it hard to estimate earnings.