I encountered some major hassle at Stansted Airport when I tried to get on board with my camera rucksack plus the "kangaroo bag" I was wearing around my waist to carry my purse and boarding pass and documents. I was told I was "only allowed to carry ONE item on board", and it would have needed to be either the camera rucksack OR the "kangaroo bag", even though the latter was almost part of my clothing. I decided to squeeze my purse into my camera rucksack and put the "kangaroo bag" into the suitcase to check it all in together.
Thus "fat", my rucksack definitely no longer went into of those devices that tell you that "only if your hand luggage fits in here it may be carried on board". Those devices are only made for quite rectangular bags, like business cases, and large ones at that, mind you!, but not for something as bulgy as my camera rucksack had become with all the things squeezed inside, too, that had been in the "kangaroo bag" before.
So the security lady told me to return to the check-in counter and check that bag in, "it must not go on board". I put up a veritable fight. Took the purse out, squeezed it into my trouser pockets, pointed to the amounts of space on the left and the right in that "measuring rack", told her I had gone on board other planes (and from the self-same airport only days before) WITH the camera bag and I WOULD GO NOW. "Pffft," she went, "they won't let you pass customs, you just wait and see!" and I replied "Watch it, they won't care!".
And they didn't.
But there was NO SUCH THING as "one item PLUS camera gear". Nothing to be had of THAT. (OK, but that was three years ago. Not sure about how they deal with things today).