Can you wait?
I have no skin in the game so my opinion would be to wait for the car you wanted in the first place. You've detailed why the USB interface is important, but only you can determine if the your ownership lifetime use of the USB interface gives £700 worth of utility.
Is it worth 38 pence a day (using the car every day for 5 years)? Then wait and get the exact car you wanted or you could be kicking yourself every day you drive your not-so-ultimate driving machine?
In either case- you should ask for something from the dealer who bodged up your order- you may not get a price reduction but shop the accessories catalog and ask for a throw-in.
If the car is coming to them and they have to carry the car a while before it might sell (you didn't detail the rest of your build), the sales person may be
VERY motivated to get you to take that car at a discount.
If it was me, I would not take any salesperson's word that any electrical components of the car are retro-fittable without official word from BMW that it is possible, despite what they tell you. Ask for it in writing if they insist they can do it. See where that gets you
Good luck!