Unless you are driving over because you are transporting a pet then it is best to sell your RHD car in UK and buy here for unless you are very fortunate to sell to a returning Brit RHD have no value in Spain.
The cost of matriculation of RHD car is about 1,000 euros . it is not necessary for your car to be converted to LHD although the headlights have to be changed and you have to have owned the vehicle for 6 months.
Bring all pertinent documents relating to your ownership of the car with you.
If you start the process of matriculation immediately BEFORE registering with your local town hall then you will avoid import duty.
One has 30 days to complete the process so using a gestoria or lawyer is advisable.
Point to remember that when you tell your UK insurance company you are relocating to Spain your insurance will be converted to 3rd party after, in most cases, 30 days.