I wonder if any of this traffic will be going up the M6 now?
Probably depends on where the pax are going and how they choose to access the airport.
Can't see much advantage in driving north from MK/Northampton, etc., just to fly south again. However, Dublin and Shannon might be a different matter if Ryanair serves these destinations from BHX.
Travelling by public transport from MK and Northampton to BHX in theory should be very easy i.e. train to BHX International Station. However, it's as easy from MK to LTN by simply taking the Railair Link coach.
My bet is that most of Ryanair's BHX pax will probably originate from the West Midlands and from the M6 corridor northwards from there, rather than from the south midlands area.