I know this wasn't aimed at my post(s), but it's quite obvious they aren't doing it to spite Northern. They deal with what they have. Stagecoach also have EMT, which suffers from 158 stock shortages (mostly down to the DfTs preference of London Midland in the Central Trains split) on the rammed Liverpool - Norwich service.
I will be the first to admit that it's annoying to see SWT operating a flashy 158 on an 11 minutes shuttle service, when 2+ journeys have pacers on Northern. However, it's not SWTs fault, is it? Look at TPE having to use a 185 on the Windermere - Oxenholme Lake District shuttles, a very lovely 20 minutes ride, but it doesn't warrant a Desiro. So, there are examples of this in both areas.
I agree - the Windermere branch is an equally daft example of a TOC using a "one size fits all" approach and using something far too valuable (a three coach 185 in the Lake District, a 158 on the electrified branch at Lymington)
At the same time we have London Midland and Scotrail using 170s on routes that 150s would be suitable for, yet other TOCs have to use unsuitable stock (Pacers running anything over half an hour, FGW using a 153 from Bristol to Cornwall).
One easy suggestion for SWT would be to wait for the Thameslink cascade when Southern should have a lot of stock freed up (since additional Thameslink paths south of London will directly replace some Southern services, meaning fewer EMUs required)... that allows SN to give up a 455 to SWT (which already run 455s, admittedly not in Hampshire).
You could even argue for 313s to be allocated to the branch (the lack of toilet shouldn't be a problem on such a short journey...).
Mind you, what I'd really like would be a new order of SWT EMUs to allow a number of 450s to move to London Midland/ West Coast (for Manchester Airport - Glasgow/Edinburgh) and Northern (to cover new electrification in Lancashire, plus replace the 1980s EMUs which could go to NXEA/ LM)
(alternatively, why not give SWT a small batch of Pacers for the Lymington branch and the "Salisbury Six"? You know it makes sense...
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What were the original reasons for FGW's disposal of the 180s? Were there any apart from the obvious reliability issues?
I'd presume that the availability of HSTs (some of which they purchased outright) meant that the 180s looked ripe for replacement