Perhaps the case for EC - although both C and J are valid booking classes in 'Business Class' on BA - both are bookable as CE or CW and there are fares which require these classes in both 'products'.
Yes, that's quite true, but it's missing the point.
The letter codes are used for fare buckets ("booking classes" if you will) but there are also single-letter codes representing the cabin of travel. This is two quite distinct uses of the letter codes.
It isn't just "Perhaps the case for EC". You can see these class codes (NOT fare buckets) on the boarding pass stub (not the PYOB kind). For example,
here you can see a class code of C - the flight is to Malta so it's Club Europe.
This is a Club World boarding pass - it always has a J regardless of the fare bucket.
Poring over endless online images of boarding passes (why the f**k am I doing this?) it seems that this is all correct, except I may have been wrong about World Traveller. It seems that BA actually uses the letter M for this, the same as for ET. I haven't travelled in WT for about ten years, so do forgive me.