After reading the Telegraph article Bill of the Hamptons posted, the proximate cause of this T5 mess is now obvious.
BAA built a shopping mall disguised as an airport terminal - and produced a result that is a failure in both roles.
Why didn't BA see this coming? It's an old business axiom that you cannot be all things to all men - you need to focus on your core business and discard the rest. That's why the fashionable business conglomerates were such a failure in the 1970's.
BAA cannot run shopping malls and airports at the same time because over time, resources and energy are going to be focussed on the "wrong" projects.