I have a UK registrated yacht which has been in Lanazote for the last 9 years. We fly back and forth but the yacht stays put. Last year I checked with the marina if I might run into trouble with tax and the reply was that as long you are in transit no problem, Defination of transit was as long as I visit other
ports and keep the receipts. If your boat never leaves port you may have a problem. As regards BREXIT I don't think anybody knows, The Cruising Assoc website' keeps members up to date on regular basis