Just to stir up the LPL vs. Man thing, in my ancient BOAC 1958 timetable, certain London to New York and Montreal flights by DC7C aircraft route through "Manchester/Liverpool" Airport according to the schedules! I assume this means Ringway, as it was then known.
I disagree with the notion that Lo-Co's only serve the leisure market. EZY's success in particular has been based on the business market as much as the leisure market. Just go to LTN or LGW in the early morning and check out all the suits with laptops flying with them.
No, the UK doesn't have too many airports considering the population density and poor surface transport. The market will decide which of them are viable on a long-term basis. Surely most people would rather fly from a local airport given the choice, if it saves the hassle of a long (meaning time-consuming) drive.
Why shouldn't the northwest have both MAN and LPL airports? The extra runway capacity provided is useful, and allows for long-term growth which will surely return when this present downturn has reversed, as it will.
Any debate about catchment areas has to take into account that different routes have different catchments. For example, here in the southwest, if you want to fly from Cornwall to London, you don't want to drive a great distance, so NQY is the obvious choice, with 3 airlines and 3 London area destination airports to choose from. On the other hand, for a charter to Greece or long-haul to New York, Bristol is perfectly acceptable. Therefore, all four airports in the region (BRS, EXT, PLY, NQY) have their own markets and are all very important to the economic well-being of the region.