Firstly, if you were trying to "bed in some piston rings" then accelerating at full revs is a sure way of knackering things up big time..
Secondly, the police would only try to sieze your vehicle if you had no insurance or some other seriouis offence, they would do it there and then.
If I was you, I would take evidence into the station to prove the engine had just been fitted with new parts and state that you would not jepardise the engine by driving the way you are being accused of.
I am confused as to your original post because you say that thery tried to sieze it at the time, by this I assume you mean at the car park, and yet you say they have just been round saying you drove off at high speed, surely you were not that good a driver to have lost them within the space of a mile.