I remember something that Easysjet said one of the reasons ordering Busses was the possibility to go for larger planes.
Keep in mind that larger planes allow not only to cope with slot restraints, but offer also lower CASM. AFAIK the improvement is approx. 10% by each step (318->319, 319->320, 320->321). So, IF U2
succeeds to sell that many tickets for one flight it's highly recommended to operate larger planes. They operate some relatively long flights, e.g. Germany/Britain - Greece. Also here larger planes might be useful. BTW, Germanwings operates a mixed A319/A320 fleet, too.
Maybe, Easyjet is able to pick up some 2nd hand A320s, too?