Mullard primarily was a component manufacturer. It was bought by Philips at a very early stage and in a sense the company still lives on as a part of first Philips Semiconductors and now NXP Components although the name ceased to exist in the '80s. Many of the early valve types from the '20s are called PMsomething (for Philips-Mullard).
EDIT: To drive off-topic an off-topic discussion, here's a nice pictorial on the manufacturing process of a Mullard EF80 valve - I'm sure even an electronics heathen will appreciate the complexity! This is a fairly new and common valve type you would likely find in the last valve TVs. Now imagine the nice ladies at Mullard Blackburn factory still churning out 27 million valves per year in 1974...