Theory and practice do not always match.
A British citizen with a right of abode has a legal entitlement to enter the UK whether or not they have or show a passport, whereas they have no right to leave without a passport (for historical reasons leaving the country requires the consent of the Crown).
Is it not curious then, that the passport checks are on inbound rather than outbound passengers ? Before I am flamed, the reasons are obvious, but it is - from a legal standpoint - a curious anomaly.