Done this several times now, the trip north on East Coast was fine, found an unbooked table seat and enjoyed the comfortable trip and free wifi on the Edinburgh-Glasgow leg, although the platforms are numbered odd at Waverley !
The northbound on TPE was also nice, seats comfy for a long trip, bar the lack of seat booking options via our work ticketing/travel system - found a table seat with ease though.
Southbound at 12:40 is an altogether more horrid experience, the Virgin/TPE option, tiny windows in the 390, a rush to swap trains at Preston onto the busy (from) Blackpool train, the 16:20 Man-Shf train was surprisingly less busy, southbound on the XC 22x was really awful, seats are nasty on a long trip, SMS seat booking didn't work, lots of family's and kids, no trolley until Newcastle !!! The best returns really are via East Coast with some shorter use of XC 22x from Newcastle if necessary. Practically impossible to use a laptop at anything but a table seat, shame the wifi on the long distance routes is so flippin expensive :-/