With seat reservations!
2 adults and a child on the 1133 from Berwick to Derby (but, obviously, you could get off before Derby and/or only use one adult ticket).
The biggest catch is: you'll need a family rail card (and requisite child!) for them to be valid. Oh, and no refunds as they're cheap tickets (hence them being on here, lol!
£20 posted for the three or WHY?

