10mm would be better, and 400mm centres are fine if using 10mm min, direct to studs is the normal method, i would place a few noggings where boards join as well, and tape the joints too.
They can hardly let under 18s vote now, Labour have plans for a curfew on under 18s using some social media/sites, so obviously they think they're not mature enough to make their own decisions (y)
Nearly 2 yrs old, looks like it has been sat around in storage for a while? you will find a lot of similar aged, very small mileage EV vans around, many pre registered to meet EV targets.