When looking up tickets with national rail you can check train times with a via option.
Is this just for planning purposes or is it possible to buy the via ticket?
Any ticket you buy having used that search will be valid for that journey. In some cases this may mean you actually have to buy two (or more) tickets, because the "via" station would not normally be on a valid route.
Note that if you purchase advance tickets, you are not allowed to leave the train (or at least the stations) except at the end points specified in the ticket.
If it shows a price, then you can get a ticket; this might be two separate tickets if it's not an approved route from A to C, or it might be a single ticket - either an "Any Approved Route" ticket, or a specific routing if there are specific routings available.
If it doesn't show a price, then you can't get tickets for that journey and you'll have to break it into pieces yourself and buy tickets for the individual pieces. This usually happens if there's a very long delay between changing trains, or if you've done something silly like London Euston to London King's Cross via Edinburgh.