It's worth mentioning that a squad does the same HE damage regardless of its HP. So a 1 man rifle squad will deal more damage than a full Bren Group. Because of that, Bren Groups are effectively worthless for actual fighting. They are however useful in a few roles in the Scottish division because of their cheap price:
1.) Sending ahead of normal rifle squads during the initial deployment to act as bullet sponges for aggressive pushes. They are in the same truck as Rifles, so the enemy is likely to fire at them with panzershrecks and AT guns, revealing their positions. It's not a big deal if they are lost while in the truck.
2.) Holding ground.
3.) Forcing enemy units to surrender while in the truck. Yes, that's right. Don't unload them if you want to use them like this. Wait for an enemy unit to start routing, and then fast move the truck forward to try to force its surrender.
I don't recommend getting them for the Guards because they don't have enough infantry slots, and while they might be worth taking in C for SSB, I would rather have the assault pios that only cost $5 more.