If you did buy a LHD in the UK you will still have to buy right dipping headlights and get the vehicle itv,d(equivalent to mot)and re registered. You will also have to have fitted a rear fog light on the left hand side if it has not got one fitted already. The tyres must be the same make and size on each axle eg both Michelins on front and something else fitted to rear or the same tyres all round.
We bought our RHD vehicle over three years ago and got it re registered, tho I now believe the authorities are clamping down on this due to the drivers impared vision being RHD.
There are many people who can re register your car for you at a reasonable price, where are you thinking of moving to in Spain??
Im sure someone else will reply to you as this is a very helpfull forum.
Welcome to EOS and good Luck with the move.
Steve.