Hi @Michael.
firstly welcome to the Outlaws.
There is no such thing as a trivial or stupid question. I have a 2.8m tender powered by a 5hp tohatsu which is used as a tender to the mothership. Also as a fun river toy for my lad.
The boat does not need to be legally registered, it's only registered with the manufacturer for its warranty. If you're buying an older boat without warranty then no registration is legally required.
There is the "small ships register" which every small boat owner is asked to register their boats on. But, again this is a best practise exercise and not a legal requirement. However the future may see this change!
Hope that helps.