The cops gave you a problem. They gave you something that was not theirs to give.

If it had been me then I would have said "great, now I have to go and get legal advice". I would have refused them.

But at the same time what am I supposed to do? Unload them in the impound lot and drive away? That'd piss off the cops, who could possibly fine me for flytipping.

It's a shitty position for the cops to put you in.