If EZY did start operations from LHR, i'd imagine there would be a lot of route cannibalisation with other LON airports. I could see business orientated routes leaving LGW/STN and moving to LHR, then leaving room in other airports for new destinations or increased frequency on the more popular leisure orientated route.
The airports fees would be the scary part, but there must be a premium they could exploit by going the LHR. If BA can get A320's in/out, surely EZY can