[JAS84]

This isn't just something that came along with digital TV of course. BBC One was originally called BBC Television Service, abbreviated on screen as either BBC or BBCtv. When BBC2 launched it became BBC1. And that was in the 1960s! But it's not that simple for ITV. The reason ITV didn't become ITV1 when ITV2 launched was because nobody would've called it that, the regional names were still in use at the time. By the time they did add the 1 in 2001 (when ITV2 introduced it's second, matching, logo), they were making plans to phase them out, so people would come to know the channel as ITV1. In theory. Turns out people still called it just ITV or the regional name, so they switched back when the current logo was introduced in 2013.