It's about 240 miles by rail between MAN and EXT. At Intercity speeds it should take just over 3 hours. It actually takes over 4 by the few through trains. The same as it takes to drive.
It's not the time in security that puts people off. (As I write the queue in EDI is 3 minutes). It's the need to arrive so far in advance of departure. And the tram or bus takes 30 minutes to reach central Edinburgh. Given the choice of an uninterrupted train and a plane journey in several bits, I'm sure the train has most of the GLA/EDI to BHX market. The fact is that domestic traffic from EDI/GLA is static or falling, and must be hurting Flybe.