In one sentence...
It is currently required by UK and International law to advise your website visitors that they are being tracked with cookies, what information is collected and in which ways that information is being used.
In a bit more detail...
As a response to the E-Privacy Directive of 2009 the UK made amendments to the Data Protection Act and The Privacy and Electronic Communications Regulations 2011.
The current status is therefore that there is no requirement to explicitly "opt in" - presuming it is made clear, continued browsing counts as consent.
CANDDi now also offers a link to that you can add to your website to opt out of tracking (see how to set this up in CANDDi).
Conscious Solutions (our lovely legal client with an excellent example): https://www.conscious.co.uk
Both recommended by clients, these guys offer free privacy policies!