There's no need to be discreet (or discrete, for those so inclined), BA mainline policy is that customers are permitted to consume their own alcohol onboard. They will even happily provide you with a free plastic cup from the BoB trolley to enable you to do it in a relatively civilised manner. If the crew become concerned with how much you have drunk they will ask you to stop - at this point it becomes an offence not to comply.
I recently had a chat with a CSM who happened also to be a crew trainer about this (he watched me bring out a speedbird ale from my bag!)... he said that on BA at least there are far more 'valid' customers wanting to do this than problems with those who do, so they see no need to ban it outright. And long may that continue.