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Reg Plate Prefix depends which location you buy

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No - it is just assigned and given to you.
DVLA gives each dealer a set of regnumbers to use. Dependant on the throughput of the dealer this could be anything from 20 to 200+ at any one time. When they register a vehicle the dealer chooses a number from their set of allocated numbers.
A lot of dealers will let you look through their available numbers and choose one you like. You would usually do this on a visit to the dealer days weeks or even months before you collect your car.
Since you don't do any of those pre-collection visits with Tesla sounds like, from what you say, that they just pick one at random and assign it to you.
 
"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.