What Halfords have on the shelf as BRG will be one specific BRG and it says in the shop somewhere what paint code that relates to. If that's not the right one for your car then it doesn't matter that it's called BRG. Just because it's off the shelf paint doesn't mean it doesn't have a paint code.
Checked my paint code - it's RCR. I found a site which lists the meaning of Rover paint codes, and it appears that RCR is a combination of HAM (BRG 1) and NAL (White Diamond II).
Gonna phone a paint place later to see if they can do me aerosols of that. Fingers Crossed!
