Where someone is found to be selling unstamped containers (over 35cl) of alcohol (of the types referred to in Schedule 2A), or possesses or transports such containers,with a view to their sale, HMRC are entitled to seize the containers (and their contents) and initiate a criminal prosecution. This appears to be a 'summary only' offence, which would thus be heard before a Magistrates' Court.
The maximum penalty is a level 5 fine (currently �5000), together with the confiscation of the containers and their contents.
Such a prosecution would not, of course, prevent further prosecution for indictable offences related to evasion of duty, where appropriate.
Chris