If you want to register a car an agency will charge you around €450 which includes the transfer tax and MOT plus you will have to change the headlights plus the brands for the tyores must match for the MOT
Your are only legally allowed to run a UK car on UK for 6 months per annum then you are iegal and if stopped they will take the car
The prices of 2nd hand Spanish cars are still more than in the UK but have reduced dramatically in the past 6 months so look at these as well