The issue with cabin baggage problems on Flybe is the type of aircraft they fly - Small and narrow Dash 8's and EMB's, even the ATR
None of these have standard cabin baggage size overhead lockers, like the 737 or A320 family (used on most Loco airlines) where a normal 55x40x25 type pull along fits OK in the bins which most folk buy thinking they are cabin suitable - (BA Easyjet WIZZ allow 55x45x25)
So unless you are a Flybe regular and know the sizes and the smaller a/c types they use then you may get clobbered unless you pay attention to the size of your own bags.
Flybe is 55 x 35 x 20 cm,plus another small item (laptop handbag or briefcase)
Flybe operate a very ''business'' oriented product and therefore the businessman wants to take his bags on board - to apply fines like this is pretty crass IMHO and just stresses the pax ground staff and ultimately the cabin crew