The 1821 is a Philips full wave rectifier on a B4 base with a brown composition base. The UU5 is given as an equivalent but has twice the rating, so the UU5 may replace the 1821 but not the other way round.
See1821 for internal details and 1821 for a shorter envelope version.
The dome of the envelope contains the etched lettering. It is unusual to see a Philips valve showing as Made in England, possibly this was made for export.
Another example but one where the internal structure is more visible. The filament looks to be made up of several strands of wire twisted together. This valve has a maximum diameter of 52 mm and is 115 mm tall excluding the base pins.
The balloon envelope is 52 mm in diameter and, excluding the B4 base pins, is 106 mm tall.
Reference: 1043. Type 1821 was first introduced in 1930.