I think you're being overly cynical and not considering operational realities here.
The practice of making it impossible to book tickets online by making trains reservation compulsory in the timetable data, and not releasing any reservations is unfortunately common place. It is very clearly an attempt to put people off; I've fallen foul of it myself, and it is really frustrating to end up having to go around ticket offices until you find one that is either willing to sell a ticket or explain why I can't buy a ticket for a train that is planned to be running. Avanti were especially bad at this, but unfortunately they are not the only ones.
Can 9 coach services even fit on the platforms/in the passing loops at any stations on the West of England line?
An alternative, more honest approach would be to make very clear that trains are expected to be very busy, suggest people reschedule if at all possible, and give them the ticket flexibility to do so. That final point may seem obvious, but I have been told by a TOC that I should avoid travelling even (they put out "Do Not Travel" warnings), but that listening to their warning would require me buying a new ticket (I had an advance on a train that wasn't, at that point, cancelled), so apparently it is necessary to point that out.
Do people really want to go from Exeter to London via Southampton taking more or less 4 hours?
If there's no other route, people should be given the choice, journey planners do tell you expected journey times.
Are there enough 159s to run an hourly 9 coach service from Exeter, plus the Andover-Basingstoke shuttle, plus the Romsey loop services? (I believe there are 29 159 sets available, 7 of which are ex-158s with less power.)
Are you saying there should be a through service, and if so can 159s keep up line speed from Southampton to Waterloo without messing up timing for the 444s that also have to fit in? (I suspect not)
A better solution may be an easement to allow travel via a not normally permitted route
I put it to you that SWR is doing its best with the hand that it's been dealt, save that blocking ticket sales on local journeys is poor form.
It's really bad, especially for people who don't know what is going on. This practice of preventing the sales of tickets which will often end up displayed as "this train is sold out" or "Tickets for this train have not yet been released" are confusing and it really should not be done; It may make sense if you're in the know, but for most people it is really bad
