"The MMSI is assigned to a boat"
I don't know about the UK, but in the US if the handheld is going to be used off the boat, it is supposed to be licensed as a "portable" which is specifically not assigned to a vessel, but documented as being used for transport, crewing
, and other purposes that will be associated with a user, rather than a vessel.
I've got an EPIRB
that is registered similarly: NOAA was quite happy to tell me to just indicate on the registration
that it would be used "portable" and not assigned to a vessel, and that's what they see when they pull up the registration
. No boat assigned to it, vagrant in transit, call one of the contact numbers to find out more.