The yellow T-shirts at LGW with nothing better else to do would be on a look out for large cases while the manning the security lines. Maybe this is BAA's approach to help reduce the level of unemployment in the UK

T5 have people issuing plastic bags for liquids who might keep an eye for larger cabin bags. I normally shake my head and smile politely.

Security folks only keep an eye out for bags that would clog up the security machine. I once saw a large lamp shade do precisely that at T5. How it ever got close to the security machine I will never know. It was about the size as the the mouth of the security machine and got stuck once it was inside the machine.