Just to confirm, the only way you will get a 175 into City is to chop off a section of fuselage behind the main gear (also called a 170!!)
it's not a weight issue.
Secondly. I agree a q400 is cheaper to run than an E Jet. So let make a estimate (these are guesses so please don't pull them apart!!) and say that it is £6 per head cheaper for an EDI LCY flight. If this was therefore £20 vs £14, that is a big percentage saving and worth a look. When you add APD though and this say becomes £74 vs £80, I can't see many people wanting saves those few pennies suffering the seat pitch of a q400 vs the BA 190.