The entire purpose of the British moving to, and keeping the bag charge ammunition, has been to reduce, and even eliminate the possibility of ammunition fires.
The Bag charge its self is not a Pressure vessel, meaning it physically can not explode unless it is in the breach of the gun, sealed in by the projectile, and breach block., it can BURN yes, but is not going to do a turret toss like a T-72 or other vehicle.
If anything a Challenger getting hit in the ammo rack should be more like a Abrams or Leopard being struck in the Turret Bustle and the Blow out panels working.
it should not be a immediate death to the vehicle. Especially since the ammo racks them selves are wet stowage. the Bag charges being surrounded in a Glycerin mixture designed to put out any fires if the ammo rack is breached.
The 1st complete loss of a Challenger 2 was due to the ammunition exploding, bag charges also blew and engaged emergency turret ejection systems on Chieftains in the Iran-Iraq war.
The wet stowage is to slow the burn process enough to give the crew a chance to escape if the worst should happen.
That burn out was due to the internals of the vehicle being hit by a HESH shell, directly to a open hatch, and the tank burned for almost a HOUR before the Ammunition cooked off.
There is a also the Castlemilk incident, a case of a bag charge detonating in the turret of a challenger killing two of its crew during training, but not causing any other bag charges to detonate.
I think I read the file for that, one of the safety pins was misplaced before firing and caused the whole to turret to be vented with hot fumes from a misfire. At least as I recall, it was a year since I read it. But I clearly remember it saying that everything though charred was still intact. Including the charges in question. Most of the damage was from the charge that was primed in the barrel already.