Nothing worthwhile will be achieved because :
1. There are now a substantial number of travellers with dual nationality, who can use either passport and generally use the one that gives them the less hassle, eg use a UK passport entering Britain to avoid needing a visa, but use a foreign passport when leaving for the same reason arriving at the overseas point. I've never encountered an airline reservation system which handles multiple passport data.
2. As I understand it the new departure checks will not reconcile with what the airline system has, nor with any arrivals detail from the past.
3. The biggest hole in ring-fencing the UK is the land border between Northern Ireland (UK) and Ireland, which is completely unchecked and politically impossible to do anything about. Arrive at Shannon, drive up to Belfast, ferry to Scotland.