Back when GWR had 150001 and 150002 they would regularly be used on Great Malvern to Brighton or Cardiff to Portsmouth, yet a 166 with declassified First Class would be shuttling back and forth Avonmouth. Why 001 and 002 couldn't be limited to Avonmouth or Weston-super-Mare services I do not know.
I think it was because many crews weren’t trained on them at that point so they couldn’t have worked South. There was also some clearance work needed on the route, namely the tunnel under Dunas Aqueduct (between Bath and Freshford), Portsmouth and Southsea high level platforms and various other locations.
That said, the 150/0 twins, as well as the 150/9 twins (when they had them) would still operate these services presumably because they had nothing else available and nothing they could swap with en route.
Even worse was when they used to send single 150/1s or 150/2s which were totally inadequate capacity wise. On a couple of occasions, single 153s operated but had to be terminated at Southampton due to not being cleared to go any further.
Allocations definitely used to be more inconsistent before the Turbos moved across. Once a single 153 operated (starting short) at Westbury to Cardiff Central and back to Bristol where it was finally swapped, and the 20:38 from Westbury to Bristol threw up everything from a HST with a full 8 carriage rake to a single 153. 143s have also run from Westbury to Exeter, St James Park to Cardiff and Exeter to Penzance in recent years.
Nowadays, short forming is still common on GWR but at least it’s not quite as extreme as before.