UK Airforce:

Code consists of letters followed by digits. The letters have the following meanings:

VCR - valve cathode ray (CRT)
VGT - valve gas triode (thyratron)
VI - valve indicator (tuning eye)
VR - valve receiving (general RX valve)
VS - valve stabilising (gas stabiliser)
VT - valve transmitting (general TX valve)
VU - rectifier

UK Army:

Again, letters followed by digits. The letter part gives the valve type as follows:

ACR - Army CRT
AR - Army RX valve
ARD - Army RX Diode
ARDD - Army RX Double Diode
ARH - Army RX Hexode
ARP - Army RX pentode
ARS - Army RX Screen Grid (Tetrode)
ARTH - Army RX Triode Hexode
ARTP - Army RX Triode Pentode
AT - Army TX valve
ATP - Army TX Pentode
ATS - Army TX Screen Grid (Tetrode)
AU - Army Rectifier
AW - Army Stabiliser

UK Navy:

Another Letters followed by digits code. The letters give the type of the valve, as follows:

NC - Navy CRT
NGT - Navy Gas Triode (thyratron)
NR - Navy RX valve
NT - Navy TX valve
NU - Navy Rectifier

Others:

CV (common valve) numbers replaced the above 3 codes during WW2. There is no way to decode these.

BVA numbers (British Valve Association) were assigned to valves used for civillian replacements in WW2.