There really isn't any need to book - if you go to the RailTrack website - http://www.railtrack.co.uk/ - and enter your trip details into the trip planner, it will give you the times you need. You can then just purchase your ticket when you get there, the trains are rarely full (except for the peak commuter trains).

I recently spent several weeks travelling in the UK, using a BritRail Pass, and the only time I booked was when I used a sleeper.

BritRail Pass would be the way to do it though, it is so much cheaper than buying point-to-point tickets.

Dave