Just wondering what the difference in ticket price between say the West End to LHR on the tube and the same West End to BHX on HS2 and tube or taxi. My guess would be a multiple of at least 4 for the latter, and given the vagaries of air travel timings you'd better have a more flexible ticket for the return at the usual much higher price. First Class on the very expensive Heathrow Express would probably be much cheaper!