How easily could 8-car trains feature (using SDO where needed) if the future fleet order includes 4-car units? While some routes do need a higher frequency, others primarily could do with longer trains. I'm thinking the Bristol-Bath-Westbury corridor needs this most: if Cardiff-Portsmouth could be run with 8-car throughout this will be of huge help to this corridor, as longer trains would both hold more passengers and reduce dwell times through more doors provided, especially at stations like Bristol and Bath Spa.
The plans for extra stations along South Wales main line seem to have 170m platforms (good for 8-car 20m EMUs), although they wouldn't be long enough for 8x23m (which would need to be 189m long), although has been suggested any Bristol service would be class 197s
On Bristol-Westbury Corridor
Bristol Temple Meads platforms : 1 is 96m, 2 is 161m, 3 is 297m, 4 is 116m, 5 is 152m 6 is 168m, 7 is 154m, 8 is 147m, 9 is 232m, 10 is 161m, 11 is 223m, 12 is 162m, 13 is 281m 15 is 276m (so some can take 8car)
Keynsham platforms are both 209m
Oldfield Park platforms are both 129m
Bath Spa platforms 1 is 195m, 2 is 279m
Freshford platforms both are 121m
Avoncliff platforms 30m (operated as front door opening only)
Bradford on Avon platform 1 (northbound) is 120m, 2 is 133m
Trowbridge platform 1 (northbound) is 121m, 2 is 154m
Westbury platform 1 is 224m, 2 is 315m, 3 is 295m
....and for information (stations beyond Westbury)
Frome platform are only 109m
Dilton Marsh platforms only 27m
Warminster platforms 1 (northbound) is 128m, 2 is 121m
So some platforms could just about accommodate 8x20m, fewer can handle 8x23m without selective door. Some platforms can already take 8car trains. Wouldn't really be worth extending platforms at minor stations (Avoncliffe, Dilton Marsh, Freshford)
If Bristol-Westbury locals could be 6-8 car, these could divide/join at Westbury to send/receive one unit to Salisbury, and another to/from Frome/Yeovil/Weymouth, which may allow these extensions to have a doubled service frequency while retaining the same frequency from Bristol-Westbury.
Plenty of trains attach and detach units at Westbury, (often used to move extra units to/from Bristol, so not aware of any problems.
As can be seen from list above 6x23m (which requires 143m platforms) only requires a few short extensions to operate without selective door.