Just a thought but was it a car supplied to the feds by a manufacturer for 'evaluation' as a lot of forces seem to be trying out high performance cars to use on Motorways.
They've had 'fast' M-way cars for years, Granada 3,0 and 2.8i, Rover SD1 and then Cosworths and the like. An old friend of mine got an ex-police Triumph 2.5 Pi, that was quick.