You have to remember that places like Leeds has Bradford next door to it with no real end of one place and start of the next., like wise Newcastle has Gateshead. Remember that LOndon is actually that big, you have to take into account the City of Westminster etc. So I think that to decide on population sizes for airports it is easier and more acurate to say out to a radius of x from each airport of drive time.
Devonair CVT will not handle as many passengers as MME, BOH or EXT for many years to come if ever. DSA may take them over next year although it'll be tight and if not next year will probably do it the year later.