£1 each way National Express tickets

I haven't used Megabus as such because they often drop-off some distance from the main destination and their timetables don't usually allow you to take Daytrips. Saying that, I've found that if you book to go from London to Oxford with Megabus you actually travel on the same groups Oxford Tube service (4 buses an hour each way).
Also, beware about going to Milton Keynes!!!! The dropping off point is at the MK Coachway (just off the M1 and 2 miles from the town) for both National Express and Megabus. There is, apparently, a bus service from there to the town, but it costs of course.
As an alternative, the Easybus service (London-Milton Keynes, seats from £1) takes you directly to the town.

Just to make a comment on some things said above - it is true that some seats at £1 on National Express are at silly travelling times, but as I said originally, if you are flexible and book reasonably in advance you can still get the seats at good travelling times (Just like the budget airlines). And yes, it can be difficult (but not impossible) to book more than one £1 seat at a time, but there is nothing to stop you from making more than one booking. On an early evening trip back from Norwich there were only 7-8 passengers and only 1 had paid the full fare, and that was because he bought his ticket from the driver - another 1 had paid £2, the rest of us had only paid £1 each.

The best rail deals I've found are if you can travel from 14days on from the date of booking.
See https://www.thetrainline.co.uk (you'll have to register to get the prices) for UK travel, https://www.raileurope.co.uk for Europe and https://www.nationalrail.co.uk (https://www.railcard.co.uk is the direct link) for money saving UK railcards.

I did say that you would have to register with the trainline site, but don't ask me why. Maybe it has something to do with being able to buy the tickets online!!
I registered with the site nearly two years ago and have not been pestered with any emails as the consequence. I've just checked my account details with the site and seen that the only details they have on me are my name and email address, so not a problem for me.
The savings to be made depend on where you want to go, and when. I've just looked at London Euston to Manchester in about 3 weeks time and seen that the basic (that's without a rail card) Saver Return Fare is £55 but it's possible to buy the same journey on 2 single tickets for £24 (£12 each way). Quite a saving I would say.
Of course these fares would be available in advance from the station but, of course, you have to get there and then hope that the ticket clerk knows what you're asking for!!
This particular journey is operated by Virgin Trains who, obviously, also offer the same deal, but you'll find that you'll still have to register with them to find out the details (funnily enough the details that you have to supply to them will also work on the trainline site, and visa/versa)