Not to argue the point, but I've had a passenger who challenged this (at LGW South) and the BAA Zulu One eventually gave in and permitted the item through security. He argued that the item was a "musical instrument" (it was some weird set of plumbing with pipes and stuff) and those are specifically excluded from size limitations. It had to be manually inspected as it didn't fit through the X-ray but it made it through.

The airline however has no obligation to accept a musical instrument as hand luggage and we insisted on gate checking it. He wasn't really bothered as he managed to dodge an excess baggage charge.