Well, firstly I don't think Joe's lying, because he's not the type, so I don't doubt what he said happened. I've also experienced the same thing myself, although not in London. The UK is full of rather cliquey little pubs, often located in small towns or villages and housing estates, where if your face isn't known, then the regulars stare and wonder who you are. That's not at all unusual.
Secondly, I don't like Guinness, so no matter what the barman thought, I'd insist on drinking what *I* wanted to drink, providing it was available, and if it was a vodka and orange, then that's what I'd be having, or I'd be out the door pronto!
No-one tells me what I can or cannot drink in a free country.
Marco.