There used to be significant suppressed demand thanks to shortage of services, op. princess proved that. And thanks to the lack of AP fares on most XC services I'd say there is plenty of suppression of demand there. Problem comes when you look at the size of the routes though, and the difference in demand throughout them, now this is something that TPE suffers from as well, but lets not go there.
The thing is that XC are being used both for local and national services, and I don't care how many times people say "Oh but just because it runs PLY - EDB doesn't mean people use it for that distance" Yeah, just because you can go from KX to INV doesn't mean people use it, but they do!
Speaking from public opinion, a lot of pax I have spoken to (and potential pax) are put off using XC because of overcrowding or "Sitting on that small train for 6 hours" and what is now a massive loss of old through services (Thanks Arriva and VXC).
Personally, previous services such as those to Gatwick Airport and Brighton or other cut off destinations where very attractive to me, mainly thanks to not having to cross London, people don't mind being on a service a little bit longer if it links to an airport.
And since XC are more about cutting non-core services than adding new ones that is also suppressing demand, Birmingham - Stansted Airport for example, why terminate at Birmingham and not run this service to Manchester via Manchester Airport? There is a hell of a lot of demand waiting in the wings for Manchester - Stansted, especially with longer distance budget flights coming into there, cutting Gatwick via Olympia was just moronic, the Gays Express (Manchester - Brighton via Gatwick Airport) was rather popular with those sick of Canal Street, and ran at a convenient time too.
So yeah, I recon there is a lot of suppressed demand, both in the core, and for direct journies that have been lost.