We're pleased to advise that our booking engine at tickets.railforums.co.uk, which helps support the running of the forum with every ticket purchase, has had some recent improvements, including PlusBus support. Find out more and ask any questions/give us feedback in this thread!
Corrections very much welcome, but as far as I know, there aren't any fully offline journey planner apps left. There used to be a few to chose from, but as mobile connectivity has become better, they seem to have dropped the "offline“ element in favour of real-time journey planning which incorporate delays and cancellations.
Not strictly a journey planner, but it is possible to download the national railway timetable in PDF form onto a mobile (other almost national alternatives are available ), however, especially in the current climate, this offline document becomes out of date very soon.
CG Transit will but you'll have to throw them a few Euros and it may well fall into the usual HAFAS issue of the UK data set provided to Europe as between average, poor or awful. Cracking app in other countries though.