I think maybe a crucial point is being missed here.
Spending money fitting fire protection to the Nimrod bomb bay MR2 or MRA4 is surely like buying a gold plated bucket to catch water from a leaking roof. Fix the roof and you don't need the bucket!!!
Stop the fuel escaping into the bomb bay in the first place and the need for bomb bay fire pretection is reduced. I accept that I have simplified the process slightly but surely fixing the source of the leak is a more sensible approach.