I expect that there's about to be a detailed answer from someone about how this formally works#, and explaining that tickets are only authorised for use on permitted routes, rather than 'any reasonable route' which was (until more or less the time of privatisation) the case.
But it seems to me that in practice and for the vast majority of the time - well in excess of 99% of the time - if a route seems sensible, then you can use an open* ticket on it. Either it will be a permitted route, or any railway staff you meet won't know that it's not a permitted route, or they won't care.
So if you want to travel like an ordinary member of the public, use a bit of common sense (so Edinburgh to Glasgow via Carlisle? No, because that would be silly). If you are interested in doing it exactly right - and maybe take advantage of oddities in the railway rules of what is a permitted route, then read on.
#Oh - there already is, while I've been writing this post.
* Open as in not an advance, fixed train ticket, which you can only use on the itinerary booked.