There is a guide line. Have a look..
Updated in May 2012, our cookies guidance (pdf) sets out the changes to the cookies law and explains the steps you need to take to ensure you comply. The updated guidance provides additional information around the issue of implied consent:
Implied consent is a valid form of consent and can be used in the context of compliance with the revised rules on cookies.
If you are relying on implied consent you need to be satisfied that your users understand that their actions will result in cookies being set. Without this understanding you do not have their informed consent.
In some circumstances, for example where you are collecting sensitive personal data such as health information, you might feel that explicit consent is more appropriate.