There are two types of Nimrod's here, firstly the older MR2 and the newer MRA4 which was to be the replacement aircraft.
The MRA4 is not a new airframe, but a re working of the fuselage with larger super critical wings, larger air intakes for the newer Rolls Royce BR700 engines, the flight deck is borrowed form the Airbus A340.
Its regarded as a bit of a cut and shut.
The Nimrod a reworking of the de Havilland DH 106 Comet, a aircraft that first flew in 1949.
There were a lot of problems with the new design, the most worrying being that it had "significant aerodynamic issues with flight control problems in certain regimes of flight", in other words it does not fly.
It was nine years late in delivery and £789 Million pounds over budget.
Now the MOD are looking at the Boeing P-8, a modified Boeing 737-800, (a aircraft I am very familiar with). It has the larger wings off the Boeing 737-900 with a strengthened fuselage. The Boeing 737-800 first flew in 1994.
Bearing in mind that the Boeing 737 is the worlds most popular airliner, parts are freely available and so are engineers to repair it.
The Nimrod would only have one operator world wide and spares etc would be strictly limited to the MOD, meaning that BAE could charge anything they wanted.
The cost of the Boeing is expected to be £1 Billion as opposed to the Nimrods cost of £ 3.6 Billion.
http://e-goat.co.uk/wiki//index.php?title=Nimrod