Originally Posted by
carsnoceans
I know BA doesn't release award space to Expertflyer but do they release seat availability?
Yesterday, I was told by a BA agent that a flight was completely sold out including J cabin. Whereas I could see I and R fares available on EF.

Yes (for revenue bookings).
Fares are different to availability. Whether you can see a fare doesn't mean you can buy it or apply it to a particular flight. A fare (or a fare component) is completely disconnected from a route or a particular flight. They are different concepts in the travel agent language.
You need to check the availability on the flight. Use the "Flight Availability" option.