I have to agree with Qwerty' If you look at the larger picture; and realise that the Authorities have to make sure they subject all their attention on making sure they do not admit persons that might provide a threat to the UK.
How the infrastructure for this, is provided is down to the Airport Operator. It would appear, that they supply less resources to this, than
providing a shopping mall experience for departing passengers. ie. no profit for them!
If X amount of flights land and need those amount of passengers to be processed at such and such a time; who is responsible for that!