The 6GW8 was one of the last audio triode output pentode valves designed. Uses were in domestic radio and audio equipment. Normally in single ended output stages for radiograms and tape recorders. The design proved popular in low cost stereo systems and was made by several companies, this exhibit being labeled Clairtone.
Clairtone is not a name associated with the UK so the made in Britain wording indicates the Clairtone were buying them in and judging by the top of the envelope Mullard were supplying.
The triode section looking through the carbonised anode to the grid within.
The thin glass tube envelope is 21 mm in diameter and, excluding the B9A base pins, is 70 mm tall.
References: Datasheet, 1040 & private communication. Type 6GW8 was first introduced in 1961.