Hi Jean,
You only need residencia in Spain if your home in Spain is to become your MAIN residence.
If your main residence is in the UK, where you live and work, then you don't want to become resident in Spain.
As I said in the other thread 'New law about buying a car in Spain' from what we can gather so far, you do not need to be resident in Spain in order to buy a car, but you do need an NIE, a copy of the padron certificate (if you are on the padron) and/or your Title Deeds.