Cameron's idea of "exit checks" is completely different from the rest of the world and has already been implemented. It is just a more thorough database to collect API and puts the responsibility for collecting API onto transport providers.
I don't think there was ever any plan to reintroduce staffed desks or build automated gates to systematically check identification documents when exiting the UK. For one thing it would require a major reconfiguration of most airports, as it would in the US, since currently immigration control is performed by airline gate agents. (Domestic and international flights depart from the same secure area but if flying domestic, your photograph is taken when going airside and compared to your face when boarding)
I seem to recall being on a tour bus via Dover ferry, and having to show my passport to both UK and French immigration control. Could be mistaken though.
I know it's unlikely they'll retrofit UK airports to allow physical border control outbound, but likewise am sure this is frequently considered/suggested.