It`s up to the sender to mark up the envelope - if they mark it up as a gift, then the limit increases to £36.
However, they do still need to give a clue as to what`s inside (DVD is classed as `optical media`).
For the full low down on importing goods, check out :
http://www.hmce.gov.uk/bus/regions/dutyrate.htm (for duty rate calculations)
http://www.oft.gov.uk (for your legal rights on buying from abroad)
Hope this helps, Julie