I believe that they definitely have room in the off-peak. As for the peak the capacity bottleneck appears to be the size of the apron. Aircraft are only allowed to stand at the terminal for 30 minutes. There is also a weekend curfew where no flights operate from Midday Saturday to Midday Sunday. I see from other threads on here that Airbus is going for A318 approval at LCY
. However I have no idea how many airlines might switch to such an aircraft.