If Flybe do use an E175 or E195 into Heathrow, their landing charges will be double those of a BA Airbus A319. The landing charge at LHR is calculated by taking the margin versus the Chapter 3 or 4 certification criteria for an aircraft of that size. The BA A319s achieve a 20.1EPNdB margin, placing them in the "Chapter 4 low" category at £880 for a daytime landing. The Flybe Embraers achieve 14.9 or 11.1 for the 195 or 175 respectively, putting both in the "Chapter 4 high" category at £1,760 for a daytime landing. With nine landings a day, that tots up to just short of £3m per year in extra landing charges for the Embraers versus a BA A319.
Using the turboprops - which fall in the same noise category as the A319 - is the least bad option. 78 seats for £880 = £11.28 per seat for landing fees. For the E175, it's £20 per seat or E195 at £14.91 per seat. Ouch.
Still not sure it's a sensible move on the Q400, E175 or E195 though!