The long standing voluntary Coast Guard Identification Scheme, CG66, for yachts is being replaced over the next two years by the RYA Safe Trx system, which runs on an app.
The detailed information is on a web page which is unfortunately not recognised as safe by some web browsers so it is attached as an image below. The official information is provided by the Gov.uk site and the MCA site.
You can also link to the RYA information here.
NB If you want to use the app to report safe arrival, it ONLY WORKS WHEN YOU HAVE A MOBILE PHONE/DATA CONNECTION! (And if your mobile phone is working!) So please do not use the safe arrival function if your destination is out of mobile phone and data coverage, or if there is a risk you won’t be able to keep your mobile phone charged up. You are pretty much OK on the east coast, but many anchorages on the west coast do not have mobile coverage.