I've had a look at Belfast International Airport (BFS) which is the next busiest UK airport after BRS - it's approaching 6 mppa. Interestingly, easyJet, Ryanair and Jet2 have presences as have TUI and Thomas Cook. Wizz has one destination and a number of other airlines operate summer charters as they do at BRS. easyJet is the biggest presence there with 18 routes plus 14 seasonal routes - about half their BRS network. Ryanair has 13 routes plus 2 seasonal, again about half their BRS complement and Jet2 has 6 routes plus 19 seasonal. TUI and TCX have considerably smaller presences than they do at BRS. So these three low-cost carriers apparently can work alongside easyJet and Ryanair at one of the smaller airports in the country so perhaps it's not entirely out of the question that they could do so at BRS as well.