Any mailing address in the U.K. will do - I have done 2 registritrations as Part I instead of the SSR. The vessel will need to be "de-flagged" from the U.S. and with that paper and the rest of the documentation
(builder's certificate, measurement, bill of sale
, etc.) plus fees
sent into the British Registry you'll have your provisional or full registration
within days. The downside is you'll need a tonnage survey
and that might not have already been done.
An alternative is to get a BVI registry, which can be done in your name as you are from an EU country, or you can create an IBC and register the boat in the name of the BVI corporation, which can be a way to avoid such things as death tax as the corporate ownership
can be passed on without paying the U.K. taxes
on the value behind those shares.