It's the only long haul trip the Airbus fleet do, and you have to be specifically trained to fly the A318 for the steep approach into LCY which means BA only train a small group of pilots. In essence the A318 is a sub-fleet of the main Airbus fleet. There's nothing special about it (despite what the boys will tell you as they wave their fancy silver lanyards ;-) ) but I suppose there's novelty involved if it gets you out of Europe a couple of times each month.