I had a similar experience on a bus one afternoon. I was at the front, upstairs, travelling along Piccadilly, when a bloke plonked himself next to me, pushing me a bit to the side (he was a bit portly), then whipped out a smelly meat pie which he ate, scattering crumbs all over the place. And his personal hygiene was none too good either. There were plenty of empty seats on which he could have sat without discomforting anyone else. Fortunately, he alighted fairly quickly, otherwise I would have moved to another seat.
I do not sit next to, or indeed close to, other passengers on buses and trains if there are sufficient empty seats; I think it's just good manners.