BA should police HBO better as it appears to be an untapped market for them to potentially make more money. If people are abusing oversized baggage as HBO, they should simply make them pay a surcharge at both check in and at the gate if they have lied at check-in and booking, with a higher penalty being at the gate as they have had the chance to review the terms and condition both at booking & check-in.
I am fortunate as I never need more than a standard size rucksack with me onboard. As it will almost always have my laptop/tablet and always medication so it can never be checked. Whilst it can fit the seat in front I would still utilize the overhead bins for extra leg comfort.