Andy Burns wrote:

Historically you tend to expect gas to be charged by volume,

No. Historically gas was sold by the therm, which is a unit of energy,
not volume. A therm is 100,000 British thermal units (BTU or BThU,
depending on the age of the reader[*]). There's a fixed conversion
factor between kWh and therms - one therm is approximately 29.31 kWh.

Conversion from gas meter readings to chargeable energy units has always
required use of a 'declared calorific value' figure, which varies
depending on the exact constitution of the gas. 'Twas ever thus.

[*] When I wer'a lad, one had to write BThU for British thermal units.
BTU meant Board of Trade Unit, which, very confusingly in the current
context, is a synonym for kWh - it was the 'Unit' used for billing
electricity. So in days of old (and not that long ago actually) you
could have said correctly that 1 BTU = 3,412 BThU. But time passes and
the both the Board of Trade and their unit have ceased to exist; BTU and
BThU seem to have become synonymous.

--
Andy