"I suppose that's a good business strategy," said Izzy, turning around to look at the speaker. It's worth noting that, until he'd come to London, the total sum of his experiences with foreigners was Mr. Theralt, who was Canadian and the only person he knew who was paler than him. Thus, anyone from any kind of exotic locale, especially if they looked like they were of a higher class than him (which this man did), made him incredibly nervous that he would manage to insult them somehow and face some kind of dire consequences.
Holding up his glass, he replied, "Already got some, sir," he lowered his voice a little, "Not sure about it, though. If I was you, I'd avoid taking drinks from anyone, or anything what's been sitting out." It was about then that he realized how stupid that sounded, and quickly said in a more formal tone, "Sorry," and explained weakly, "I just have a bad feeling about it."
Izzy hoped he'd just get a raised eyebrow and a change of subject, or perhaps the man would leave. He wasn't sure how he'd explain it without saying 'There are vampires here and one of them might have bled into it. It might be a bad thing if you drank it.' That probably would have been the only thing that would have made his previous statement sound sane.