The UK domestic business class fare, as stated, entitles you to use the Terraces lounges and be flexible with your ticket. But not to any other seating than 3+3 mixed in with everybody else, and the Bistro Bag.
A day or so before the flight, and indeed at the airport on the day, it may also be the only class offered. I have taken many BA domestic flights (I book a lot the day before) which are "Y no availability, J available" which turn out to have more empty seats than can be accounted for by no-shows. Aer Lingus are even worse in this respect. It afflicts the business peak flights more than middle of the day.
Furthermore we sometimes get quotes in the office for both J and Y on the same flights which are only say £2 apart on approx £250 fares. I had to explain this one to our accountant too !!