Guiness (excellent name)
The low-cost market is not as straightforward as your question makes out.
For example, Ryanair have staked their sector of the market by operating from large Population centres (Dublin and London) to smaller airports, some close to major cities others that are marketable. easyJet and Go, on the other hand, have initially concentrated on the SE UK market to major destinations. The imporant part is to anchor part of your network at major population centres.
The second factor is to produce a product that fits the market. Again Ryanair, because they have extremely low costs at some of their European airports (Hahn for example) can pitch their fares at the leisure market, but stand little chance of attracting business traffic on such routes. Go and easyJet have gone for Bigger airports in an attempt to capture some of the business traffic.
Now it is not all as black and white as that, but , initially you need a good population catchment area at either or both ends of your route.
Once established, both easyJet and Ryanair have ventored further afield to build their business. Ryanair to Scotland, easyJet to Liverpool. Go will probably follow suit next year with a UK base. But these bases cannot support the level of services that Low cost airlines usually like to put in place, namely 2 or 3 rotations per day to the main destinations. So the growth potential from Liverpool, for example, is limited.
The real growth area is Europe. At the moment, the percentage of passengers in the low cost sector in Europe is around 4%. In the US , it's around 15%. The predicition for Europe over the next five years is 12%. As low cost airlines are at the moment restricted to UK and Irish operators, this offers huge potential.
I see a carve up of the UK in terms of bases for the low-cost carriers, followed by a similar carve up of Europe. Already Ryanair are looking at a German or Scandinavian base. easyJet are starting in Amsterdam and are already in Geneva. Go are evaluating a number of European bases.
So, to answer your question, you will see some growth of low-cost carriers into the northern part of the UK, but of a limited nature. Manchester would be too expensive for a second base.
H