Yes, UNC means no wear but BU (as introduced by the Royal Mint in the 80s I believe for their UNC year sets) has come to mean no wear and full lustre. A tiny weeny bit of toning could possibly still scrape in as BU if the mint bloom is still there, but a coin with a finger print or any other area where there is no lustre or uneven lustre cannot be BU. There are some coins on that list that are practically completely brown with lustre in the lower parts and they are still described as something involving the term 'BU'. I hate to moan about it but I've had issues in the past with 'Colin Cooke' grading. Don't complain Neil, it's just an opinion and I'll leave the link to your website above!