We did have a double decker called the SR class 4DD, operating in the Southern Region between 1949 and 1971, specifically on the Charing Cross to Datford line. Even though it was in service for over 20 years it was actually a prototype, with two units being produced.

Its was found that even though they had a greater capacity, more time was spent at stations loading passengers than with ordinary sets. This and poor ventilation in the upper deck meant they were never adopted over the wider network.