The 7H7 is a variable μ amplifier pentode. It is a high impedance device with a medium mutual conductance. Variable μ devices are primarily designed for IF and RF amplifiers in radio receivers. The anode current is quite high and with the large electrodes, this suggests that the device may have been designed for low noise input stages of HF communications receivers.
The 7H7 design dates to 1940 and was originally made in the USA, in the UK introduction was in 1947.
The Brimar logo and Made in England.
The outer cylinder looks to be the screen.
The wide glass tube envelope is 28 mm in diameter and, excluding the B8B base pins, is 56 mm tall.
References: Datasheet & 1040. Type 7H7 was first introduced in 1940.