They don't with BA, they do with Easyjet. I recently flew out from Gatwick who didn't care at all about the size of my cabin bag (the same one I take onto BA), but on a flight back from IBZ I was stopped and told it had to go into hold, had to pay at the airport and it cost around 50€.

The irony is though, I've seen bigger bags being taken into BA cabin. Some people are starting to take small hold bags into the cabin. On my flight out to MYK last week one lady in economy had such a large 'cabin' suitcase she kept getting it stuck between the rows whilst trying to drag it down the aisle.