HELP with BOAT registration in UK (Porto)

Hi, everybody:
I need some info on registering my 19 footer Sea Ray in England( it is still registered in NJ,USA); I was told it would be easier and cheaper than here; Boat is in Porto, will be ridden in the Douro river and some little sea, for cruising,sightseeing and maybe fishing;

I´m a retired mechanic,have plenty of time, just finished rebuilding the 3.0 Mercruiser engine,want to enjoy this summer in the water ;

I checked with Capitania do Douro yesterday,they said it would be acceptable by portuguese law; if you are from England and know about this,your input is appeciated; thanks