We are not aware of any reliable data on the prevalence of Flash cookie usage in the UK. However, a recent study by researchers at the University of California, Berkley, USA (available here) found that 54 of the 100 most popular US websites used them. Of those 54 websites, only four sites mentioned the use of Flash cookies as a tracking mechanism in their privacy policies. Further, of six randomly selected US Government websites, three were using Flash cookies to retain the personal information of users.
If the use of Flash cookies without disclosure to users is as prevalent in the UK as in the US (and, moreover, if the UK Government are making use of Flash cookies as their US counterparts are), there may be cause for concern.
Regulation 6 of The Privacy and Electronic Communications (EC Directive) Regulations 2003 governs the confidentiality of communications and states that an electronic communications network may not be used to store or gain access to information in the terminal equipment of a subscriber or user unless the subscriber or user of that terminal equipment:
1. is provided with clear and comprehensive information about the purposes of the storage of, or access to, that information; and