In my experience it very much depends on a number of variables:
Plane arrives late – they have 10 mins to get everybody off then on again – you could take a grand piano through as hand luggage.
Plane arrives early – they apply all the rules with enthusiasm, and stand there making ladies stuff their handbags inside their cabin bags, and checking all the wheelie suitcases for size, then charging the people for putting them in the hold if they transgress.
First flight of the day – it can go either way !
I usually bring the biggest carry-on bag I can get, but soft sided. If the flight is full and they know the last 30 folk to board wont get their bags in the overhead lockers, they start confiscating hard case bags. If you have a soft bag they let you stuff it under the seat (I have short legs and tiny feet).
The way to beat easy-jet charges: don’t pick your seat, don’t queue for anything and listen out for the “free hold luggage” announcement. Take a soft bag with room for your handbag or manbag at the top. Board last. Its my personal airport challenge to swan on the plane last, looking relaxed, casual and unhurried. Sometimes it becomes a competition.