I once read somewhere on the internet saying that, at a pay-on-exit car park, you have no obligation to pay for parking and they have no right to stop you from leaving. The logic being that it falls under the law on private clamping and it's unlawful to block your access to the highway. You just drive up to the gate press the intercom and tell them to let you out. The attendant (if there is one) is likely to refuse initially, but they know this law and will ultimately let you go if you stand your ground, especially if it's busy and there's cars queued up behind you they'll cave in pretty quick. Additionally the barriers are designed to collapse/move easily (in case of emergency) so you can just drive through them. Needless to say I've never had the brass neck to actually try this, nor have I been able to corroborate it, but I do think about it every time I go to a multistorey, especially at the airport!