Afternoon Pete. I assume that you are also British so I just thought I would echo your comments. It always has astounded me that, in th U.S., even if you are sitting on a bar stool and the barman simply pours you a drink from a pump (tap) right in front of you he expects a tip!!! Do be serious!!! Even the Europeans have got this worked out; different prices depending on whether you sit down at a table or drink at the bar.
Just to put your mind at rest I doubt if Inland Revenue is quite that naive. I know that they have arrangements with various employees that they know earn large tips e.g. airport porters, cabbies etc. and charge an assumed amount of additional earnings. Does beg the question of course whether the same system exists in the States and if not why not.
I'm off on the Island Princess in March so I hope that was not your ship! Before then I shall no doubt visit some of our local pubs and enjoy drinking at the bar without paying for the privilege simply the drinks.