Skipness
I agree in todays aspect that its more likely that 'Midlanders' will choose to go south on HST2 than vice versa, however LHR is currently operating at near max capacity. The current government have put a block on any extension at LHR including a 3rd runway so by the time HST2 is supposed to be operational (2025?) LHR will be well and truly maxed out.
So which airports in or near to London could take up the slack? LGW?
With the HST2 it puts BHX effectively in Zone 5 in London (about 45mins away). This is incredibly convenient and attractive to customers and airlines alike.
I doubt you could get to central London comfortably and easily from LTN, STN or even OXF in 45mins or less? but I stand to be corrected as i'm no expert on Londons roads or public transport, but I do know how congested and potentially stressfull it can be.
IMHO when LHR finally reaches full capacity and London / Great Britain PLC starts to lose out big time to other major european hubs then the government will quickly do a u-turn. I envisage a new airport being built somewhere in the Thames estuary Hong Kong style but cheap. But hey thats a long way off.