Whether you are allowed to break your journey depends on the ticket type. In general the more flexible tickets such as Anytime, Off-Peak or Super Off Peak tickets allow you to break your journey. If you possess one of these tickets you may start, break and resume or end your journey at any intermediate station along the route of travel - unless the ticket restriction for the journey you are making does not allow it.
If you have an Advance ticket, you may not start, break and resume, or end your journey at any intermediate station except to change to/from connecting trains as shown on the ticket or your travel itinerary.
The terms and conditions of each ticket type can be viewed when making a booking.