You will, or should have been give a blue log book, just like the green logbooks we had in the uk for cars prior to the registration docs you now have. just send that to Department of Transport, Douglas Harbour control, Douglas IOM
and state in your letter that the rib has been "exported to the uk" no other info is required and you may find that the DOT have been informed of the sale of the craft to save the DOT invoicing for slipway fees.
Anybody else looking for a rib, the prices are lower here than in the UK
I took my last rib to Northern Ireland to get a better price.