Actually they do that. Don't forget that GCHQ steal a lot of stuff as well and then retrospectively classify it as well.

If you read the actual court judgement (which I would recommend everyone to do, normally very interesting and shows the depth to which these things are considered) you will see:

Mr Oliver Robbins, Deputy National Security Adviser for Intelligence, Security and Resilience in the Cabinet Office, indicates in his first witness statement (paragraph 6) that the encrypted data contained in the external hard drive taken from the claimant contains approximately 58,000 highly classified UK intelligence documents. Many are classified SECRET or TOP SECRET. Mr Robbins states that release or compromise of such data would be likely to cause very great damage to security interests and possible loss of life.

So the US documents are now suddenly UK documents?

I call copyright infringement on grand scale. Authors' rights are internationally protected by the Berne Convention for the Protection of Literary and Artistic Works. If the UK is claiming authorship of documents created by US citizen, the UK is either commit a crime or breaching international treaties.

The documents are UK classified that doesn't mean they created them necessarily. Indeed it has no bearing on authorship. For example a document on which a dictator of another country gave their plan for a nuclear strike against their enemies would likely be classified Top Secret as soon as a copy were possessed.

Yes, strictly speaking there is likely to be copyright infringement when a UK agent, say, copies a foreign document. But then there will also be treason, trespass and other similar crimes being committed [along with breach of other treaties than just TRIPs] that are also necessary if agents acting for the UK [or any country] are going to carry out such work.

Espionage is generally a crime.

Just to point out: If they were created by the US government , they would normally be automatically dedicated to public domain, so there would be nothing for the UK to infringe.

There are weird exceptions here, and i can't remember which agencies fall into them.

Authors moral rights are a bit more complicated, and I do not know how the UK deal with it. It is uncertain if I can in UK law take Shakespeare works and publish it as if I was the original author.

It is also uncertain if you can transfer moral rights. As I understand it, it is a legal gray zone to claim someone else wrote a copyrighted work.

>I can in UK law take Shakespeare works and publish it as if I was the original author //

It's fraud - http://www.legislation.gov.uk/ukpga/2006/35/section/2, cf. Section 2(1)(b)(i).

Could it be possible that documents created by the UK security services are shared with their US counterparts?

Is that to be believed, in light of lack of evidence so far?

What evidence could they provide of this that would satisfy people?

At this point, anything more than completely nothing, nil, nada, would be an improvement. Whereas there is a parade of unsupported statements by state security officials that have been revealed, within a week or two, to have been a lie. The fact that the burden of proof seems to be heavy is not for no reason.

It might be an improvement, but it would not be enough - so what's the point?