Best place to buy any single malt Scotch (including Highland Park) - the duty-free at London Heathrow Airport Terminal 5...best selection, free tastings, decent prices, frequent specials.
Highland Park is from the Orkney islands off the north coast of Scotland...another good single malt from the Orkneys is Scapa - just finished a bottle a while back. For a blend that uses a lot of Highland Park, try The Famous Grouse (my favourite blend) - its principal malts are Highland Park and The Macallan.
