Beautiful picture but unfortunately the details are wrong. This is the legendary Canterbury built in 1929 and scrapped in 1965. She carried the passengers for the Golden Arrow service in great luxury. The picture is actually taken from a British Railways poster produced in 1948 on the resumption of the service after the war. Science and Society also have this in their collection at the url below. I have emailed them advising them of their boob.
http://www.scienceandsociety.co.uk/results.asp?image=10173400
The Canterbury which preceded her is almost certainly the one to which Dave1 is referring and was built in 1901 and scrapped in 1927. She was one of the fleet of little cargoships which the SECR, latterly amalgamated into the Southern Railway, used to shuffle cargo between the continental ports and the UK whilst the passengers were carried on the dedicated passenger steamers. Today the cargo travels in the artics on the RORO ferries.
They were very small and the Canterbury had a gross tonnage of 561 tons and was twin screw. She was able to carry a handful of thirty passengers. Here is a postcard of the slightly earlier single screw Maidstone, built in 1899 for the LCDR. I think she would be very similar in appearance.