It's possible to get into the UK without a (UK) passport check if you go via Ireland. They treat the Dublin ferries and planes like domestic flights, they still check the passport at boarding and one presumes with it being computerised they still know you've entered the country (unless of course someone does what this guy just did).

There were signs up in Manchester that if you didn't have a UK / Irish passport (we travel on AU passport) that you're supposed to approach someone, but no-one guarding the exit. Ha ha honour system.

Doubly so if you drive across the border from Republic of Ireland to Northern Ireland, since they're legally not allowed to based on the Good Friday agreement.

In some ways this is less what we want because as tourists that spend a bunch of time in the UK I actually want someone to tell me I'm complying with the law.