Most UK based webmasters use the BBFC rules to determine what content is allowable, anything allowed to be printed in an adult magazine is typically considered ok.

Also, the cases involving retail have specifically involved DVD/VHS/Movie titles that had no classification or, were classifified as R18 which can only be sold in licensed sex shops in the United Kingdom.

Here is a link which should be of some use to you...

http://www.bbfc.co.uk

You can probably also find some information on the UK Customs and Excise website when it comes to importing adult products..

http://www.hmrc.gov.uk

Regards,

Lee