To be fair, this is probably true of some slot on the LBa-LHR route.

But have you ever tried getting a cheap flight on the early within a month of departure. (I couldn't get on the flight last MOnday or Tuesday am) Or any night on the 5pm flight. Used it again last Wednesday and they had a waitlist for the 5pm flight.

The EMB is clearly too small for some of the slots, but the right size (or possibly a little to large) for others.