There is a really good discussion about LHR T5 in the British Airways forum:
http://www.flyertalk.com/forum/briti...arisation.html
Note message #14, written by corporate-wage-slave who has exceptional knowledge of the BA and LHR ecosystem. In this message, he/she discusses Ready to Fly Conformance. In other words, you must pass Conformance a certain time before your flight departure, else you will be offloaded. Read this message and the whole thread carefully.