Q. Is Heathrow the only international airport that prohibits you from carrying more than one bag onto a plane? Even if your one piece of carry-on luggage is accompanied by nothing more than a tiny purse (let alone a normal-sized briefcase) they make you check your bag. It seems outrageous.
A. London's Heathrow Airport does not have a rule limiting carry-on bags to one item per person, although it did at one time as a result of heightened security measures. However, your airline can impose its own limits (both on the number of items and on their size and weight) and perhaps that is what you experienced. Many airlines now weigh carry-on bags at check in (or even at the gate) and have maximum weight restrictions as little as 10 or 12 kilograms (or 25 pounds or less). I recently flew on an Airbus A380 and noticed a warning on the overhead bin that the maximum weight for the bin was 50 kg, so perhaps that's the reason for weight limits (maybe the overhead bin would break or fail to operate properly if there were too many heavy bags).