Row 4-7 probably blocked as BA havn't decided on the required config for the day, or they are blocked for silver and golds. You may be able to move further forward when on-line-check-in opens at T-24.
Don't forget that the "B" seat is convertable, and I don't find it that comfortable... feels marginally narrower.
I would prefer the other side of the plane; If the flight is not full, could always take an aisle and window and hope the middle seat is left empty...
Some A319's have slim line seating and reduced pitch behind the exit row (30-31" as opposed to 34" in front of the exit. Catering / drinks should start from the front of each cabin.