BHX don't own the trolleys - the whole system is run by a company called ACS. They provide the staff and equipment.
It's a possibility that BHX charge ACS rent and ACS recoups it via the £1 charge- maybe somebody who knows can confirm this?
What happens when the contract comes up for renewal and ACS have made a loss? Does BHX suddenly become trolleyless?
On a related point, has anybody noticed the addition of a change machine in T1?
S78