I’ve been doing some digging into this as I have an interest in getting one myself. It seems that European law, which we are still and will probably remain subject to, states that the number plate must extend beyond the rear wheel when sold new. This means that it will not pass the factory pre delivery inspection or MVSA test unless the holder is in such a place. Once you have it you can make amendmends within certain parameters. It must be near vertical and as close to the rear as possible but it doesn’t state that it has to be at the very back. It will pass a MOT like that providing the plate itself is legal but that’s a separate matter. Weirdly it’s not permitted to be on the right hand side but I can’t find out why.
Hope that helps, it’s certainly reassured me.