Do NOT use self tappers as they will pierce the skin of the van and render your warranty invalid.
Get the proper number plate fixing pads from Halfords, make absolutely certain the back of the plate and the area of the van where it is being attached to is perfectly clean and free of grease, polish etc by cleaning with meths, then apply the pads to the plate and then to the van.
Press firmly for about 20 seconds to ensure a good bond, and make sure it is done on a dry day.