"Being hopelessly non-mechanical myself, is it fairly simple to swap the number plates after delivery? Just a couple of screws?"
Can't be certain about a M3 but when I got my first MS I completely effed up the number plate swap. Phoned Digbeth SC and they sorted it out and put the new plate on for me F.O.C while I waited, about 30-45 minutes I think.
My own dealings with Digbeth have nothing short of exemplary on the following:-
1. Number plate mess up as above (by me)
2. Windscreen Wiper replacement after mess up (by me again!)
3. Front passenger exterior door handle not presenting (not me this time!!!)
4. Number plate swap on old MS in preparation for new MS
Always very courteous and accommodating when I've phoned and usually next day appointment.