ATNotts there are no regulations that you must carry a passport to leave the UK at all.
The UK does not currently operate any exit controls, cepting for juniors considered at risk of abduction usually by own parents and those the social services deem at risk of FGM typically traveling to certain African countries in the main (this may well change in the future - indeed the UK GOV have tried to implement exit controls/counting before however both cost and the illegality within the EU scuppered those plans dead)
Indeed legally you don't current need a passport to travel within the EU even outside of the schengan areas or between the CTA between Eire and the UK.
All you need is an officially recognised ID document.
For many that's simply their national ID card or even an EU style driving license.
Now by default and classically of UK beurocracy we haven't registered our driving licenses as EU recongised documents, and haven't even managed to offer an effective ID card system compliant with the EU regulation.
The last time UK GOV tried to create an ID card system it was cancelled on cost and after public outcry. So defacto the passport (With all its costs and voluntary nature bearing in mind the state can refuse issuance)becomes the only recognised UK document by default.
Airlines and airports choose to demand official ID to comply with DOT/CAA screening regulations only and to ensure its the same body checking in/collecting boarding card and clearing those Lovely long security checks (which are primarily there to control/deny restricted materials entering the airside environment) and that finally arrives at the departure gate with requisite boarding card in hand.
Last edited by Rutan16; 23rd Jul 2017 at 16:46.