Users of passports issued by Andorra, Monaco and the Vatican can still register for UK egate use by meeting the eligibility requirements indicated below:
Who is eligible to become a Registered Traveller?
A passport holder of Andorra, Argentina, Bahamas, Belize, Botswana, Brazil, Brunei Darussalam, Chile, Costa Rica, Guatemala, Vatican City State, Honduras, Hong Kong Special Administrative Region, Israel, Macao Special Administrative Region, Malaysia, Maldives, Mexico, Monaco, Namibia, Nauru, Nicaragua, Panama, Papua New Guinea, Paraguay, Samoa, Seychelles, Saint Vincent and the Grenadines, Taiwan (passport must have a personal identification number), Tonga, Trinidad and Tobago, Uruguay
18 years of age or over
Hold a valid visa (exclusions are Tier 5 Sporting & Creative Concessions, EEA family permits, discretionary leave and leave outside the rules)
A visitor who has entered the UK from another country using an eligible passport, at least 4 times in the last 24 months in one of these categories:
Business
General
Academic
In Transit
Entertainment or Sports
Parents with a child in a UK school
A member of a diplomatic mission
A medical visitor
An ‘entry’ does not include passing through a UK airport while on the way to another country.
https://www.faster-uk-entry.service.gov.uk/about
San Marino isn’t on that list, but not sure why.