Funnily enough, there is not as much competition out of London for easyJet for Malaga for instance. Out of Stansted ej is the sole carrier on the route and only receive competition from 2 other scheduled airlines at Gatwick. There are already 5 airlines operating from Manchester to Malaga for instance.
For your information Treviso is as close to Venice as Marco Polo. Oh! and Bergamo is closer to Milan than Malpensa.
A catchment area is defined by routes offered rather than the geographical location of Liverpool or Manchester Airport. Liverpool and Manchester have there own local catchment areas for Dublin for instance but Liverpool has a larger catchment area for those wishing to travel to Poland for instance whilst Manchester has a larger catchment area for those wishing to travel to the USA for instance and so on......