Firstly I feel the disappointment for you guys buying a very exclusive exciting car that appears to be unroadworthy in the eyes of the law.
Surely as in the case of the 4/4 Speedster they should be withdrawn from sale (or recalled) if they have a non functioning handbrake, it begs the question were these cars Type Approved under the +4 general banner and not in their own right.
I sincerely hope the MMC takes this seriously and as a bare minimum notifies you as to their intentions reference the solution and a loan car whilst they resolve the design error.