The Network Rail website gives details of assistance available at the station and has links to check what facilities are available at each station, on the train and details about which powered wheelchairs and mobility scooters can be carried on trains.
GWR have also created this Station Information Booklet, which contains details of the facilities available at all rail stations managed by GWR (updated July 2018).
GWR and South Western Railway have both produced leaflets explaining their commitments to making rail travel accessible, including details on arranging support, where to find information and arranging replacement transport. Links to these are below:
South Western Railway – Making Rail Accessible
You may also find this page on Wheelchair Accessible Taxis and Private Hire Vehicles useful.