Interesting thread, I have a few observations
1/ The only safe way to proceed with Ancestry (and the Internet as a whole) is to assume that anything you upload is accessible to other people, sometimes a few, sometimes very many. If this bothers you, don't upload your tree.
2/ You don't need to put a copyright notice on photos that you have taken, it is automatic in the UK. That said to gain any protection from copyright law requires deep pockets and specialist lawyers.
3/ As others have said Ancestry's private tree setting is no protection, they could change the terms tomorrow. It is also not retrospective so that making a tree private after it was public only helps going forward. If other people (or Ancestry) have copied your stuff you have no control.
4/ Think twice before uploading any images of documents that you have bought to Ancestry, you will almost certainly not own the copyright on them and you have already explicitly given Ancestry the right to use them by accepting their terms and conditions. The privacy setting is irrelevant here.
Really point 1 is all that matters, the rest is cautionary. It's unlikely that point 4 would ever become an issue for the general user, but it's not impossible, a lot of Ancestry's terms are designed to indemnify themselves from any liability by passing it to the user and they can afford much better lawyers than you or I.
PS There used to be a program that could copy other peoples trees from Ancestry, they issued a Cease and Desist and it went away but I imagine similar things are still floating around the Internet.