Last time I went Salisbury to Portsmouth the 3 car train was full and standing with cruise passengers heading to Southampton, so not a problem unique to those up north
This very discussion with
GWR▸ on Tuesday afternoon. Before
IET▸ timetables came in, there was an 11:01 Westbury to Portsmouth service and an 11:11 Westbury to Southampton with an extra stop at Dilton Marsh. The question asked was "why" and the answer was the
HST▸ that called at 11:04 from the West Country, together with the significant flow on cruise departure days when the 11:11 could be full (yeah, I know it was often a 153 so limited capacity).
It's not only a peaky traffic that differs from day to day, but also people want to leave Southampton before the next lot arrives, so even an enhanced service going down there and then coming back up later in the day is ill-fitted. Personal advise if you're amongst the 5000 arriving in to Southampton in the morning on Iona is to take your own luggage off and make for the 08:23 which starts from there.
There are plenty of good (and awkward) business opportunities for the railways at present - but they are cost rather than balance sheet driven, so not allowed to invest "x" pounds in meeting requirements even if that will generate"10x" income. Add to that the reputation for unreliability due to strikes, the reputation for fares always being high, and the opportunities that there should be are frankly seen as distractions in a few quarters, or unmeetable frustrations in others.