At the moment the JAR-OPS 0.O2 promille limit is largely unenforceable in the UK. The current requirement in the Air Navigation Order is simply that a member of crew shall not "be under the influence of drink or a drug to such an extent as to impair his capacity so to act". There is no limit set in legislation and there are no powers to require to require crew to provide samples of breath etc for testing.
This will be changed by the Railways and Transport Safety Bill introduced in the House of Commons earlier this month. Part 5 contains clauses which establish an alcohol limit of 0.02 promille for flight and cabin crew and ATCOs and provid the police with the power to require samples (breath, blood or urine) from anyone suspected committing an offence.
http://www.parliament.the-stationer...040/2003040.pdf
My understanding is that the 0.02 promille limit is higher than than the level of alcohol that can be natuarally produced by the body in most circumstances.