Advice needed buying a second hand car

Depends on how much you want to spend,new or used.I would rent a car to start just to see how the land lies and take it from there.LHD in UK is more expensive than you might think.I have just imported and registered a RHD car but it was something I especially wanted,it’s not a cheaper option.There are many cheap lease deals advertised all the time in Spain,if I wanted a new car that is the route I would take(subject to finance acceptance)Used cars in Spain are much more expensive than the equivalent in the UK and often dented,scratched and scruffy.My opinion is based on working in retail motor trade all my working life until retirement at 57.

Many thanks that’s very helpful we do prefer leasing our cars but because we won’t have our property or long term rental we believe we cannot buy or lease 

As a UK resident, you cannot drive a Spanish reg car that belongs to you on UK roads, you would have to have it transported to the port. The same rules may apply in France, you would have to check.
Buying a Spanish reg car is a minefield in Spain let alone in the UK because finance, fines, unpaid road tax etc remain with the car.
A few months back someone posted that they had done the same and found they didn't own the car, had been driving it illegally, there were debts attached and an embargo on the car. The UK car sales insisted everything was above board.
If your UK car has a decent value, get a sale agreement in place with a UK garage with the agreement there will be "X" extra mileage, drive it down and use it to find a Spanish reg car in a garage here and then have your UK car transported back to the UK.