Visiting United Kingdom
Monegasque nationals are not required to obtain a visa if their stay in the United Kingdom is for a duration of up to six months and if they have no intention to work.
Only a valid Monegasque passport is recognised as an official document by the UK Border Agency (UKBA). Monegasques are considered as non-EU citizens and cannot enter the country freely.
For stays exceeding six month or if you intend to work in the United-Kingdom, a visa is required.
Consult "Living in the United Kingdom"