If you are fully vaccinated you don't need a test to enter Spain. You just need to complete the passenger locator form and they will scan that and take your temperature after passport control at the airport.

You need to inform the car hire company that you are crossing the border (there will be an extra fee and insurance to pay).

If you are immediately entering France from Spain, i.e. not staying in Spain first for any length of time, then you need to follow the rules for France as though you were coming from the UK. If vaccinated that is a sworn declaration that you have no covid symptoms or in contact with a covid sufferer over the previous 2 weeks.

If unvaccinated I don't believe you can enter France from the UK (via Spain) other than for essential reasons only.