Thanks for the clarification, stifle!

Oh god, yes. But as they get out, the railway companies cotton on to it, and introduce specific exceptions ("negative easements") to the routeing guide.

For example, it was only recently that Cambridge-Ipswich tickets were banned from being used via London. Or Newcastle-Carlisle via Leeds. Newcastle-Dunbar via Edinburgh used to be a fun one, too...

There is another loophole, in that you can always use a ticket on a direct service between two points, however indirect a route it takes.