BVA was something like "British Valve Association" - and was where you used to send any faulty valves for replacement under guarantee - either Mullard or Mazda.
I never really understood what BVA were, but I don't think they were a manufacturer?.
The numbering on those valves is 'Pro-Electron', just like used on European transistors.
E - means a 6.3V heater
L - is an audio output pentode.
C - is a triode
B - is a diode
3 - is an octal base.
So EBC33 is a 6.3V heater, diode triode, with an octal base - the last number just signifies the particular valve type.
8 - as a base, means B9A, the most 'modern' of the valve bases, although octal was still used for larger power valves like the EL34 (6.3V heater audio power pentode, octal base). EL84 is a smaller audio power pentode, with a B9A base.