The National Rail journey planner: personal experience suggests that for anything other than a straightforward trip, the current implementation struggles. Useability wobbles aside, it would be good to know if the issue is with the journey planner, or the principle of being able to buy a rail ticket between any two stations on the system, or the underlying data, or something else.
I should avoid the example I've just checked - Bath Spa to Kidderminster - as people have helpfully picked the issue to pieces about this time last year - for that one the planner returns a list of times but also 'Not a permitted route', and against each option 'This route may require multiple tickets. Please re-plan your journey' - and in any case fails to find the optimum-priced travel via Bristol and Worcester. BRfares.com does list through prices for that journey, but again, doesn't find the route via Worcester so perhaps this is the underlying data increasingly failing to reflect real-world travel needs.
Mark
https://www.nationalrail.co.uk/https://www.brfares.com/