[Stooky Bill]

I think you're close to the money, the 21st anniversary logo from 1989 is clearly the inspiration or offspring of the wider rebrand, the differences being a 'Thames' wordmark vs a stylised river Thames.

I'd suggest there was a wider rebrand about to occur and the ITV corporate style came out, which they used for a bit - they probably did double down once they'd found out they'd lost their franchise but it wasn't a case of getting on screen elements and music redone - different tracks probably used to add variety seeing as their only symbol was the same ident each time.

The main difference between the 21st logo and the one introduced in 1990 was the change from orange triangle to blue. It was part of a much wider company rebrand, presumably in  preperation for the license bid and the new decade 

I once saw a tape in their VT area in the mid 90s labelled something along the lines of  'ice cream cone logo'. Whether that was just the orange logo or the nickname continued onto the blue one too I don't know