Legal & General sponsors...

Yes, when they changed from an orange logo to a blue logo they introduced a fairly cheap looking 'Thames' ident for local programming only. A few weeks after they lost their franchise they ditched the ITV stuff totally and just used the local ident

Anyway yes, Thames Weather was Norwich Union, an example of this and all their other branding in 1992 can be seen here: http://www.youtube.com/watch?v=Aehm0Mo2k-U
As to why, I suspect they either had a longer contract than L&G had or it made more commercial sense to have a different one to LWT

Yes, Thames would've been more expensive to sponsor than LWT, and it's not really that strange that they would only choose one of the London contractors.

And maybe Legal & General didn't operate in Scotland or the Channel Islands? Did they sponsor Grampian?

Yes.

I rather liked that ident TBH, it starts off well & the end part looks good although the pull back to the ident is a bit patchy. The 1989 Thames XXI logo did look better though.

As you say though it may have made more commercial sense to have two separate sponsors, even now there is two different sponsors for London Weather, LWT's is still referred to as London Weekend weather

Yes, it's the pull back that spoils it. It's not bad but could have been better, I suppose though that it was only ever intended for use a couple of times a day. I wasn't that keen on the orange logo. IIRC it was known internally as the 'ice cream cone'

The Thames pres graphics at that point are all very nice, especially the menus and slides. I wonder if they had them already made when they lost their franchise or whether they knocked them up in the couple of weeks afterwards

When I watched the Thames 1991 ident for the first time since their demise, about 8-10 years ago on TV-Ark, I thought the dodgy pull back was due to low bit-rate of video files back then. However since seeing it the other day on a very fluid capture on youtube, you kind of think a little bit less of it because you realise that dodgy pull back was nothing to do with the real player files of 1999/2000. I still like the ident though even for that "fault".

I do like the XXI logo although I think it looked better on the production slides/start of the ITV 1989 ident, mainly because the triangle was more even (i.e. it was on triangle with a wavy line through the middle rather than one triangle embedded into the other).

As for the rest of the ident package of 1991-1992, I'm not sure TBH, it certainly is impressive & looks rather good, if of it's time, today. If it was a rush job then it looked (and still looks) far superior to a lot of the current presentation packages that have taken 6-12 months to create, IMO of course.

IIRC, didn't Border use their variant of the ITV 1989 logo on theirs instead of the standalone "chopsticks in a bowl" logo?