The journey times are only half the planning problem — the other half is booking windows, luggage, and not over-scheduling a country that rewards slower mornings more than it looks like it should.
A common mistake is treating every city on this site's journey list as a day trip and stacking five or six into one week. Between arrival logistics, checking in and out of accommodation, and actually seeing anything once you get there, three or four bases is a more realistic ceiling for a 7–9 day first visit — not because the trains are slow, but because everything around them takes longer than the timetable suggests.
Two or three bases, each 2–3 nights: London, one northern English city (York or Manchester), and Edinburgh works well and keeps every leg under Edinburgh's 4h 8m.
Four bases becomes realistic, and this is where a rail pass's math starts to favour the pass over point-to-point tickets — more long legs, booked closer to departure as the itinerary firms up.
Enough time to add either Cornwall or the Highlands as their own multi-day leg rather than a stretch goal — both are real trips in their own right, not add-ons.
Advance single fares on GWR and LNER routes are typically released around 12 weeks before departure and get more expensive as each train fills up, not just as the date approaches — the cheapest advance fares can sell out on a popular departure while later, similarly-timed trains are still available at a higher price. If you know your dates and aren't using a pass, book as soon as the window opens rather than waiting.
Long-distance UK trains have limited dedicated luggage space, usually at the ends of carriages and on overhead racks sized for cabin bags rather than large suitcases — on a busy Friday afternoon London–Edinburgh service, that space fills fast. Reserving a seat is free on most long-distance operators but not automatic on a pass or an open ticket; do it at booking or at the station, especially for Edinburgh, Manchester or Glasgow departures.
Scotland's rail network (ScotRail) and the England-wide operators (LNER, Avanti, GWR) are separate systems that interconnect at stations like Edinburgh Waverley and Glasgow Central, not one unified brand — timetables, apps and even some ticket types differ once you cross the border. Wales' network similarly connects at Cardiff and via GWR's cross-border services rather than running as a separate booking system.