The train should be cheaper than normal because it's only considered as 'peak time' if the train gets to London before 10am. Look at Megabus, they do some train deals. Also, look at the coach as it may be a little more uncomfortable but it will be a lot cheaper.
Also travel cards seem to be cheaper than getting a regular ticket. If i left from reading or oxford or whatever with a travel card at around 10pm it would cost about £8.50 right?
And then get another travel card to go back to reading? Because I remember a long time ago I used a travel card to london from reading, and i was able to go on all trains there and back during the day.