As long as the trailer you purchase has a EU type approval plate on it and comes with the correct paperwork, then you will be able to get it registered in Spain so that you can tow it behind your Spanish registered vehicle. We (
South Coast Boat Trailers - boat trailers, boat trailer for sale or hire) have sold several trailers to customers in Spain and I have never heard of any problems.
In terms of the boat registration, most people I know have them registered on the British Small Ships Register which is simple, cheap and the Spanish seem happy to accept.
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South West Boat Transport
Professional Boat Transport across England, Wales, Scotland, Europe & Scandinavia. Any boat up to 50ft.
https://www.boat-transportation.co.uk