If you buy a Disabled Persons Railcard you can save 34% of the cost of your journey and the discount is also given to one accompanying adult. The railcard can be used any time without restrictions.
To be eligible for the card you have to either receive DLA, PIP or have Epilepsy, if you click on this link you can see all the eligibility criteria and what proof you need.
There is a small fee, £20 for 1 year or £54 for 3 years, you can apply online or via a printed application form, you can also apply on behalf of somebody.
For more information please see the South West Trains link for Assisted Travel on the right of this page.