One of the UK’s most influential science communicators is pressing the government to let more of its scientists speak to the media. “Some of the scientists who are advising government are amongst the best in the UK, yet, all too often, their voices are lost to the public and the media.” There is an argument for loosening of restrictions and a change in a culture that she believes can make it difficult to report the views of the country’s leading experts.  http://www.bbc.co.uk/news/science-environment-22845865
The Science Media Centre’s (SMC) stated role is to get science into the public domain through the media when controversial topics hit the headlines.

During the Fukushima nuclear accident in 2011, there was intense interest and concern about the effects of radiation.The SMC put up independent experts to do interviews on the subject with the media. Several of the experts that were brought in to advise the government told the SMC that they felt unable to do further media interviews. Some organisations contacted by the SMC claimed this was because of instructions from the government’s Cabinet Office, which was overseeing the crisis.
Independent scientists joining committees advising government during national emergencies, such as volcanic ash or swine flu, are often required to sign the Official Secrets Act and other non-disclosure agreements.

“We were told that special advisers at Number 10 thought that a short section about the economic recession and mental health would lead to awkward questions during Prime Minister’s Questions,”

In Canada, several government departments are currently under investigation by the country’s information commission for allegedly “muzzling” scientists.
The directive explicitly states that press officers should ensure that the minister is not embarrassed and that the interview is “along approved lines”.
In the UK, there is no such overt directive. But practices in the UK can have similar effects.

“There is no systematic attempt to silence government scientists, but many feel they can’t speak to the media – especially in the midst of a crisis or where the science is contested,”
“Some of these examples are a worrying sign that there has been a tightening of restrictions, and constraints on the open and free discussion of the science that I would say should take place in a more public way. So I would be worried.”