Thank you for your answer Dogwood,
Before building the car, I need to know how to comply with the english register rules as they probably will determine the engine choice as well as others components and even the donor car. So I guess it will be to late to pass the SVA because I read somewhere that it's going to be replaced by the IVA but I don't know more about it.
Your answer is helpful but it's not enough yet to begin the very first welding point. I've checked in the DVLA and in the Department for Transport websites and haven't to found the SVA1 form.
I live near Marseille (13).
Have you built a car ?