The cheapest tickets need to be booked about 12 weeks in advance of the game , I have found the disadvantage to this is getting booked up and then finding out part of the journey is a bus replacement service due to weekend engineering work which you find out about 6 weeks before the match.
Splitting the journey is cheaper really - book a London Travelcard (return to/from London and use of Underground and Bus in London) then use advance singles each way to Leeds or Doncaster from East Coast - got to book them ASAP though as the prices go up closer to the date (like now its £43 each way).