Firstly thank you for your advice bb21, but I figure I'll respond to your first post anyway because I've been writing it for so long!
There are a few ways you can play this, however I think the most important thing that you need to decide is what you want to get out of taking this further.
You could pick their arguments to pieces easily, however you need to bear in mind that if you make a big fuss about it, they may well collude with SWT to get things changed, so that the ticket can no longer be useful to you in the future.
You may want to make good use of this ticket for a long time to come, in which case you may have to swallow your pride and let them get away with it for this time. Getting the confirmation that you are right is not always the best solution, circumstances considering. At the end of the day, they issued you the compensation you wanted this time so you haven't lost out.
These are just a few pragmatic approaches to this case. It really depends on what your main objective is.
Well, to be perfectly candid, I'm not a big rail user - the reason for our travel to East Croydon was basically a one off, visiting the Home Office buildings there for an in person Visa application for my SO. The reasoning for our buying of these specific tickets was that they were shown by every service's websites as being valid for travel on those trains and were therefore versatile enough for our purposes (they were not suggested by the Poole ticket office). It was suggested by the home office to 'allow up to 3 hours', and they wanted us there for 13:05, and no real idea when we'd be going home. That time is pretty inconvenient with the myriad of restrictions put forward on travel to London: we wanted to take the faster South West train service and change at Clapham to get to the appointment safely on time without any risky changes (other route had ~4 minute windows), so we didn't just buy the cheaper Southern tickets that we could have that would (probably) have caused no issue for this return route. The application process was stressful enough - I just wanted to buy some train tickets that would get us there on time and give us acceptable options on travel on travel home, since even the more expensive SWT tickets were problematic since many of their trains skip over Clapham at peak times. Plus, if I could save enough money to buy Lunch, then all the better. Every search engine was very forthcoming with these as a valid option so I took it, not considering there could be a problem. I wasn't trying hard to weasel something out to save a few pence, and the cost of the Visa application we were making at our destination was ~2 orders of magnitude greater than any money we really saved picking these over Off-Peak or Anytime SWT tickets - we just wanted less hassle (and lunch).
And hassle was all we got.
Honestly the thing for me is that, this could catch someone just like us again tomorrow - an infrequent traveller buying tickets from a website for going to a very stressful and important meeting and having their day ruined by complicated restrictions and helpless staff who either refused to or couldn't acknowledge the possibility that we might have had a point, or that their answer was just the next on a pile of different ones. Thank goodness it was on the way home. Really just the one guy helped us out by printing out an itinerary that showed we could use the tickets we had for that 17:51 on that route (and even then he still said they were no good for the 16:48). If the train guard had decided our tickets were no good, I don't really know how far they could have chosen to take that but I imagine it would have made things a lot more complicated for us. If we hadn't been able to get through the barriers, we would have been stuck until at least 7 pm after a busy day around Croydon, and we couldn't see much evidence of any real seating in or near the station.
I just don't think that's a thing that should really be able to happen so easily. My biggest gripe is kind of that the barriers refused us entry, since our tickets were apparently valid for those trains (and apparently even for the trains up to Clapham) and I would have thought they would have known that and let us through, but I can understand why they wouldn't.
As for the loophole being useful, I'm not sure it really is? I can't really think of
much of a use case if you're required to bring a long a wedge of evidence that proves you can use those tickets to experience an uncertain delay at the station every time you travel, but then again I'm not very creative. I know it's not something I will be doing again, that's for sure. As people have noted, generally the SWT tickets are significantly more expensive than the Southern ones when travelling from my home station, Poole, to East Croydon, and if you're confident in doing so, excessing is the way to go
apparently. I have little to no confidence in this, hence buying the tickets that I did, but I think this might save you more money.
Further though, and adding credence to your point, searching for Poole vs Southampton Central to East Croydon via Barnham, while SWT and Southern do price tickets for both routes, the uh, the logic behind Southern's pricing seems to elude me, which seems like it might be a much larger and more 'useful' situation than trying to use a BE restricted ticket to return home as we did.
Poole to Croydon via Barnham with a change at Southampton vs
Southampton to Croydon doesn't really seem to make sense to me, unless it's priced to encourage literally anyone to use it instead of the SWT route. If you can save money and have Southern's staff smile and wave you through by playing your cards better than we did, then that's probably more of a useful loophole.
In terms of making use of this BE business specifically, I feel being informed that it is invalid for this journey and persisting anyway would be a very poor decision for anyone to make - If it is valid, it's only their ticketing system and being very specific with the language of the restriction code that says so - no actual human who works for Southern (or anywhere else to my knowledge) has said 'yes these are valid for all these trains' up front. I'm happy to accept that maybe they should not be valid, but that doesn't change the fact that they're being sold right now, and we were sold them as being valid.
To me, what we tripped over feels much more like a trap than a loop hole. And I'd personally feel a lot better knowing that this trap is closed off.
Really, I think, it is partly that I'd just like them to agree I'm right that you can buy them, and all the other usual blithe pleasantries you'd associate with such an acknowledgement so I can let it go. I kinda agree that the idea of changes and clearer rules, as well as better options for checking them so that people don't accidentally fall into these surprise cracks, is basically hopeless, but I can still point it out to them as best I can. The easy out for them is to just stop these tickets being sold for that route, the worst outcome of which is probably by 'fixing' the pricing for the Poole to East Croydon via Barnham route I guess? While the real problem is that everything else we had happen to us is going to play out again and again every time someone gets caught between what one system says vs what another system says, I'd settle for at least this trap being closed off.