Just as an aside - you don't actually need to sign the OSA for it to apply, it technically applies to everyone regardless, signing it is more just a way of you acknowledging that you understand you're bound by this law you're already bound by.
The person responsible for the leak (if caught) should be prosecuted.
The way the press is handled though becomes a bit muddled, on one hand in a free society you want a free press as much as possible. Obviously there are legitimate reasons for wanting to restrict the press in some areas such as protecting criminal trials, protecting people from libel/defamation and just protecting people's privacy in some situations.
Obviously you'd not expect a UK newspaper to publish say leaked top secret technical details of the UK's nuclear program and you'd perhaps expect them to have to hand back such information if they had it. Other things might be in the public interest. This leak, which has cost a UK Ambassador his job gets a bit close to a great area. Diplomatic communications ought to be protected, they're not in themselves political but are the communications of a civil servant expressing quite frank opinions and providing useful information. This isn't some evidence of corruption or wrongdoing leaked by a whistleblower, it is interesting to read the views but they're not really (IMHO) in the public interests. UK ambassador has sane assessment of trump... well no ****, you'd probably find plenty of other ambassadors; European, Australian, Canadian, NZ have very similar opinions too.