Heavens above, Nimrod 2 dosn't even have a bomb bay fire extinguisher! Does the MR4 I wonder??
The Nimrod Mr2 bomb bay is huge. When it has fuel tanks in it, taking up a lot space, the volume available outside the tanks to inject extinguishant into, to suppress fire is a great deal smaller, so fire extinguishers were a practical precaution.
I would guess that the size of extinguisher bottle(s), with pre-positioned spray heads/tubes needed to put out a fire in an empty Nimrod bomb bay would probably take up at least a third of the space in the bay. It would need a sustained blast of extinguishant throughout the bay. Is that practical? No.