Deal as below, copied and pasted from travel zoo:-
Rail tickets between London and Birmingham are on offer for just £5 per person with Chiltern Railways, a fantastic saving considering fares can cost up to £50 for a single ticket on this route.
We found good availability for midweek travel between July and September, but you may need to be flexible with your travel times to find the lowest fares. Availability is very limited on Saturdays, and tickets arent available to book on Sundays.
Travelzoo tip: You can get tickets sent straight to your mobile phone as a text message -- just select the mobile option before making payment and enter your number and make of phone when prompted.
How to book: To book tickets with Chiltern Railways, use the orange buy now button on the homepage to enter your travel dates and times.
Additional information thanks to "Pops is here":-
Fares available on routes
Banbury (travel to London or Birmingham)
Bicester (travel to London or Birmingham)
Great Missenden (travel to London only)
Stratford upon Avon (travel to London only)