If you are resident in France you have to pay the same taxes as a French citizen even if you continue to fly - be registered in the UK. You also used to have to have a survey if no other boat of your type was already registered. The survey however was paid for by the French government except the hauling of the boat. The last bit changed last January and now the survey merely requires you to have paperwork supporting the LOA, Beam and engine power, The paper you end up with is very slightly different from that of a French resident but cost the same. The paper entitles you to keep the boat in France but to conform to UK safety and security rules..

I think in Spain you may continue to fly your national flag but as a resident you need to have the survey, pay the same as a Spanish Citizen and differently from France conform to all Spanish safety regulations...

Michael