(2 part post):
Answers as per your numbers for the questions:
1. If you owed the clamping company money, they would have the same right to prevent you driving your van as a garage would have if you hadn't paid the bill for work done on it, so I can't see that they (or the police) were doing anything wrong.
2. As above.
3. Because you'd not paid the release fee, I would think that the firm were probably within their rights to remove the vehicle and charge you the appropriate fee.
4. Whether the police should have moved your van might depend upon where it was. (e.g.. if it was, by now, on a public highway, they could use their powers to shift a vehicle causing an obstruction). Even if they were acting outside of their powers, it's unlikely that you could do anything about it. There's no way the CPS would consider criminal action against the officers and they couldn't be held responsible, under civil law, for any damage which occurred once they'd moved it.
5. No. Refer to Answer 1. (Anyway, to be guilty of theft they would have to be seeking to permanently deprive you of your property. That's clearly not the case here).
6. No. At least, on the information which you've provided I can't see any evidence to support such a claim.