I have seen that in several places, including the BBKA notes. However, the Food Standards Agency "Lot Marking Guidance Notes"
http://www.food.gov.uk/scotland/regsscotland/regsguidscot/foodlotmarkguid * has:
There is a warning that the batch size may be inappropriate so it may not be the best strategy but the "best before end" format is specifically permitted to act as lot mark if it is in the correct format because it is an implicit date of the longer form.
If you can find the requirement in primary legislation, then fair enough but the Food Standards Agency official guide notes say "best before end" is enough.
*I have no idea why the version that search turns up is in the Scottish section of the web site, but it's the London contact address at the bottom and the previous archived but identically worded versions were elsewhere on the site,