only 141.000 miles and a auto for a plod car ? 
Most Met Omegas were autos

The guys talking bollux - guess he hopes it will sell his car to some poor unsuspecting buyer.
Two minutes to swap out the speedo, means nothing

speak to any race engine builder they will say that the MAX you cab get on a normally aspirated engine in 100BHP per ltr.
So a 2.6 should be no more than 260BHP (at a bloody hard push) but yes will require a lot of work at a cost of at least a couple of grand.
Some how I do not think the police will do that.
Police spec cars use same engines as road cars but the cars are lightened. maybe a slight remap but thats all.
Nope... they might be lighter as they roll out of the factory, but by the time they're equipped...
Take the Omega, most civvy facelifts have sunroofs and electric rear windows, many have full electric seats. That little lot is probably 60kgs, but plod cars have extra bracing in the roof, springs are thicker (and therefore heavier). Fully equipped, a plod car will easily weigh more than an Elite with the same engine/gearbox

That car, with wood trim and CD spec seats is almost certainly not an ex plod car, especially being a 2.6...