Episode #2.5 - S2-E5
Revealing mistake: When Ben Miller asks for the forensics report on the oyster and shot glass, the report has egregious spelling mistakes ("Specimin" instead of Specimen) and shows the victim testing positive for Oxycodone rather than strychnine! For the record, it is an entirely different lab test (a real one, with 'specimen' spelt correctly) done for a patient under prescription medications (morphine, oxycodone and benadryl) and its date is inconsistent with every other episode (it's a 03/19/2010 document and in the first season it was established the year was 2011), but these last details can be noticed only if you freeze-frame. (00:42:00)
Answer: Most likely they had only tropical exotic drinks, he wanted an old fashioned English beer.