Citizens of Monaco don't enjoy freedom of movement in the UK, and may only stay visa-free for up to 6 months.
Nevertheless, according to Timatic, uniquely among non-EEA/Swiss nationals, they can enter on a national ID card.
Do Monaco ID card holders get some sort of receipt of their legal entry (like I as a Swede get in Turkey), or does the Home Office exclusively rely on electronic records to track their status in the UK?