[Stuart]

. . . given most of the UK has transferred to digital, the need to identify the main ITV as "ITV1" just doesn't seem to make sense to me.

It's like how Channel 4 and Channel 5, for as they make sense as names, didn't need to change their main channel's name because they launched spin-off channels.

DSO completed for UK terrestrial TV broadcasts over a decade ago on 24 October 2012.

C4 use '4' as the main part of their channel names, as it's their brand. They have never used additional numerals to define their subsidiary channels.

Similarly, C5 use '5' as part of the name for many of their subsidiary channels. They have also never used additional numerals to identify them.

If you have ITV 2/3/4, it makes little sense not to have 'ITV1'.