The supplier for these isn't exactly "chosen" - there's no committee that reviews the options like there is for [-]pretty much everything else associated with the olympics[/-] choosing the host city. It's no different than any other show bid where the lowest/best bidder gets the job, but as in many other large shows, the designer can drive the choice of shop based on the gear they want or their relationship with the shop. If you want to use a Bad Boy/Best Boy/V676 on your show, then you have no choice but to rent from PRG - you can't get them anywhere else. I have no idea if that was a driving factor for choosing the London supplier, but all three of those products are being used throughout the various venues.
For my venue during the 2010 Games, I had gear from both the major suppliers in Vancouver, Upstaging, Creative Technologies and Creative Stage Lighting. My company had gear from Toronto, Orlando and Dallas, and we were a small rig.
The Opening and closing ceremonies in Vancouver was PRG, and the only thing they really subrented was cabling.