Get a solicitor. Seriously - writing letters won't help. Civil prosecution for them taking your property, complaints to the IPCC etc. will. Clearly all of this will hinge on the "good reason" you had for carrying your tools. Provided that the landowner of the wood will confirm that you had permission to carry out wood carving there, you were transporting tools to carry out a legal activity.
This sounds like the best advice. I'd also ash the solicitor about damages due to any lost job prospects, etc. resulting form the "warning' which will be "on file."
The answer may not be what you (or we) would like to hear, but you'll never know unless you try.