The size of some of the carry-ons is ridiculous. Problem with trolleys is that they have to go overhead. I usually try and eep my bag with me under the seat in front- it's usually a rucksack type bag, so fits nicely, apart from when fully packed, in which case it's a bit more of a squeeze.
That way I don;t need to worry about scrambling on board first to secure locker space. It's for this reason the I try and avoid exit rows on SH.
Ba is not unique here. I've just done a few domestics in the USA on AA and all flights seemed to struggle. Moreover the crew were generally disinterested in assisting pax.
I recently flew KL from LHR and the gate agents at T4 were very pro-active in gate checking carry-ons. I was collared as well, and when I protested (politely of course) I was told "it's a very small plane" (not true- it was a 738). I was eventually only allowed to take mine on board after promising that I would fit it under the seat in front. Once on board, I noticed instantly that the lockers were all empty. So the gate agents were too efficient on this occasion.