I assume bits of Nimrod are safety critical and include software? Stores management and primary flight reference data displays spring to mind as possible candidates.
Yes I think you can assume that Nimrod does have safety critical code and likely in the areas you've mentioned.
But to go back to your intial suggestion:
Compare now to Nimrod MRA4, which apparently has 6 millions lines of code - OK not as much as VISTA, but then again each copy of the software comes with quite a large free aeroplane. Total cost is £3.6Bn. Now this is alot of money, but I think this perception is particularly exaggerated when it equates to £300m per copy, when you only buy 12.
Those
primary flight reference data displays
being derived from Airbus. The Boeing TCS is a version of the P8 kit and various other bits of existing kit culled from here and there, and might together add up to 6 million lines of code.
But clearly not all that code was developed just for those 12 Nimrods, even if the UK taxpayer is being charged as though it was.