Getting from London to York by bus for a day trip is only recommended if you are on a strict budget that prevents you using the train or renting a car. The bus journey takes 4-5 hours, sometimes more, so is not practical if you only have one day to spend in York. The earliest bus from London in the morning won't get you to York till lunchtime.
The only advantage it has is that fares can be very cheap, so if cost matters more to you than time, you could take the bus to York. (At the time of writing there was a single fare available from London to York on Megabus for only £5.) You would, however, only get to spend a few hours in the city, which is not long enough to see its many attractions. To make the bus a feasible option you need to be staying at least one night.
Booking London-York Bus Tickets in Advance
National Express run buses daily to York from London, starting in the morning and then roughly every 60-90 minutes until about 6pm. There is then one late overnight bus. Journeys take at least five hours, often more, and some journeys require a change of buses and a lengthy wait in Leeds, so always check the details of your journey thoroughly in advance. Bookings can be made on the National Express website, which sometimes offers discounts on tickets and cheap bus fares through sales. At normal price, the bus costs less than half the price of the train fare.
The cheapest advance tickets are currently through Megabus, though there are a limited number of services and you cannot make a day trip using them. They do, however, offer very cheap tickets if booked in advance.
London to York by Bus
To find current timetables and fares for the coach services going from London to York visit the National Express website, or Megabus.