I'm going to pitch in here with a bugbear of mine. I travel lightly, a slim laptop bag which goes under my seat and a holdall with the bare essentials for 3 nights away - it's generally only some t-shirts and jeans or dirty laundry depending on which leg I'm on.
I put the holdall up top and the laptop bag under my seat
On more than one occasion, I've encountered passengers with cases the size of a house saying "why can't that go under your seat" to which I point out I already have a bag there. Much more annoying are the ones who ram their bags in pushing mine to the back because it's small.
I really wish BA would enforce the baggage rules more stringently - people will adapt.