Well - you could just tell them to get lost as there is no other possible way to transport the items ("your camera is only insured as carry-on"), you need it for your trip (freelance photographer/stock landscape photography) and the bag fits the requirements specified on the site (my "just" comment was that you would have to push the front clip into the padding of the bag by about 1cm for it to fit into the piped volume check thing - and that was only because i packed things in there tight and it was bulging slightly.) 

You should be fine with it, and any of the other smaller ones are just that - smaller and will definitely go through.  I didnt even get a second glance when i pulled a laptop out of the back to go through the x-ray machine, they are used to the bags going through it seems. I actually rang up BA and LAN and cleared it before going so i had that as a backup...

"In every colour, there's the light.
In every stone sleeps a crystal.
Remember the Shaman, when he used to say:
Man is the dream of the Dolphin"

Rich Meadows Photography