William, nothing did come of this proposal, apart from their are a couple of daily Leeds-Carlisle semi-fast services, operated by Northern. The Leeds-Glasgow through service, which was reinstated a few years ago, has also been withdrawn.
As for AP fares, I also find it strange that no singles exist, especially when the returns which are sold are generally cheaper than any walk up single ticket. When I need to make a single journey with TPE, I know normally buy a return AP ticket, which due to this system both reduces available seats on the unused return service, and possibly deprives someone else of a cheap ticket unneccesarily. To minimise the effect of this, I always try to book the wasted return portion on a service that I believe will be relatively quiet. So on the Anglo Scottish routes, are AP single tickets being sold?