Our booking engine at tickets.railforums.co.uk (powered by TrainSplit) helps support the running of the forum with every ticket purchase! Find out more and ask any questions/give us feedback in this thread!
Am I correct in thinking that such a ticket is valid for use only on the one most logical routeing KGX-SVG-PBO-GRA-NNG-RET-DON-WKF and not valid for an indirect journey such as STP-SHF and SHF-LDS?
'directness' isn't relevant, as others have informed you, permitted routes are what is relevant. Anyway, there is no sense in which St Pancras to Leeds via Leicester and Sheffield is 'indirect' - there are even a number of through trains.