Really? My hubby tips with cash. I would think some (most?) bartenders aren't allowed to drink while working.
Yeah, give him one for himself. He'd just have a half, and whilst I'm sure some bars don't allow you to drink, most others don't mind if you keep it cool or just do it when you're shutting down.
BBC, as always, puts it better than me:
"All British people know this: you do not tip cash at the bar in a pub. If you are impressed with the barkeep's service, you can offer to buy them a drink. Such an offer will be genuinely appreciated, even though it may not be accepted. If it is, the barkeep might take the tip in the form of cash to 'have one [a drink] later'. This will generally be for half a pint of beer, or a small measure of spirits - you would not expect them to take for a larger drink unless you specifically asked them to."