The airlines (or rather a select brotherhood of airlines) control slots at LHR and many other airports, through a company they own called ACL.
Much as BAA has lost the plot about most things, they are innocent in this case.
In the context of nursing slots, ask BMI the reasons for its new LHR/JER service starting March 25th. Good use of slot assets? Hardly. Keeping the slots warm until a better opportunity opens up, such as more crossings of the pond? Yup, but don't tell them in St Helier.