Eastmids is correct in his explanation of the choice of hand luggage size.
The reason for the problem, is all down to the clowns at BAA who have over the years failed to adequately invest in security hardware, baggage systems and personnel, so when the sh*t hits the fan, they cannot cope.
Their way to deal with the situation is just to restrict the amount of hand baggage that can be taken on. The size limit is not for security reasons, it is simply because the clowns at BAA cannot cope with the intensive searches currently required with normal hand baggage levels.
This also explains why you can enter the UK with larger hand baggage, which has been screened somewhere else abroad. The size of the hand baggage is not a security issue per se, it is just because BAA cannot deal with screening it. Incompetence at it's worst.
It really is a f***ing joke for anyone in BAA to say they are doing all they can. It's all about profits for BAA, security comes way down their list of priorities.
I personally, will try to fly as little as possible until someone gets the finger out, and demands BAA sort it out, and I can travel with a very reasonable piece of cabin baggage.
Honestly, this country is a joke.
Rant over.