It’s nowhere near cabin size as others mentioned and wont pass security, but neither would it pass boarding staff. This is “visibly too big” and F or no F they won’t and shouldn’t allow it to pass. on top of that, it would be grossly unfair on other passengers in the cabin. For instance on the 789 you mention, there are no luggage compartments in the central section so both window and centre passengers share the limited window side space and your bag would obviously not fit wheels first as it should but only sideways thereby depriving one passenger on your side of their luggage space entirely.

I'm not among those who think that hand luggage has to be weighted (irrelevant on BA of course) and measured to the gramme/millimetre, as airlines vary heavily and shapes of bags do too, but my limit is a pragmatic one: if it won't fit wheels first (or equivalent), then it doesn't belong in the cabin because it will deprive someone else of their space, and your proposed bag fails that homemade test emphatically.