Firstly the blame is not with the airlines, it is with BAA who insist that airlines will loose their slot if 80% frequency is not conformed to.
Just a minor point (and I'm definitely NOT one for standing up for the BAA!), but slot allocation is nothing to do with them, it's actually Airport Coordination Ltd (ACL) who are tasked with this. I'm not sure whether ACL make the rules with regards to a certain proportion of slots having to be used, or whether these come from 'on high' at IATA. They are rules that, in my view, need to be reviewed as a matter of urgency.