easyJet and Ryanair can live with each other, as the serve two very different markets - EZY serves primary airports mainly to the west Med, whereas FR serves secondary airports mainly to Italy and central Europe.
BMi Baby can survive, although I believe they may end up scrapping either MAN or EMA as they are too close to each other, and MME is a bit of a dodgy destination.
Air Scotland will be the first failure this year, the 757 will never work on budget airlines operating from places like ABZ where there is no demand, and GLA where there is sufficient charter traffic.
FlyGlobespan can probably make it work, as could MyTravel and Jet2, but the problem for these airlines is where they can create UK bases in a saturated market where all the budgets barring FR serve exactly the same destinations. e.g. EMA has quite a bit duplication from WW and EZY.
The only possible airports that could cope with direct European routes would be BFS, NCL, MAN and GLA.
How would NWI do with a low cost carrier?