Here is a link to a recent Parliamentary Written Question on this.
http://www.publications.parliament.uk/pa/cm200506/cmhansrd/cm060719/text/60719w1817.htm#column_436W

Mr. Hancock: To ask the Secretary of State for Transport what steps he is taking to reduce overcrowding on trains; and if he will make a statement.

Derek Twigg: We are currently seeing huge growth in the railways, with more than 1 billion passengers carried last year, and this is set to continue. To help meet this challenge we are investing record amounts in the network.

Focus is being given to improving timetabling, infrastructure maintenance and renewals and other factors that can impact on reliable delivery of train services, which in turn will impact on crowding levels. We are also considering different measures to increase the capacity of the network, from train layout and length, turnaround times and infrastructure improvements. Major investment programmes such as the West Coast Main Line modernisation project have already seen capacity improvements.

It is for the train operating companies to take decisions about the deployment of rolling stock. We will continue to review train counts and train plans to assess whether resources are being deployed effectively.

It would be fair to say that this issue has caused some contraversy (link below.)
http://www.saveourtrains.org.uk/