It might make sense to choose different routes in either direction. For example, I just tried some random off-peak dates and times. (I looked at travelling out, in the late morning of Tuesday 6th March and returning, at the same sort of time, a week later).
The best fares offered were �15 outbound (depart 1154, arrive 1715, changing at Birmingham) and �16.50 back (depart 1059, arrive 1624, via London - including the tube fare). I'm confident that you won't be able to beat those sort of prices, so it just seems that a bit of 'trial and error', to find the best prices is called for:
http://www.nationalrail.co.uk/
Chris