MGB fire warning is a standard manufacturers fit. You have to tell them that you don't want it. The ones I fly have always had them and they haven't caused any problems. The test is similar to the EFW system but there aren't any big red lights, just the caption. I haven't had them come on over ten years and I wouldn't like to either without any mirrors to see what was going on.
I cannot think of a situation where the MGB fire warning would come on without massive indications from other gearbox or hydraulic warnings that would have led to its landing or ditching beforehand. Two possiblities are a rotor brake fire or a severe engine fire and breakup that has fractured the engine firewall. Despite a comment about their unreliability EH still fit them so they can't be that bad. Maybe the CAA jumped the gun. We have baggage bay fire warnings so why not have one in every enclosed volatile area.