Which is the best church/cathedral in the UK ?

I second Westminster Abbey. As the official web site says it is the House of God and House of Kings. It is intrinsically bound to our history as it is where Monarchs were crowned on the coronation chair since Edward the Confessor built it in the 11th century. It also contains the remains of many prominent contributors to our land including Sir Isaac Newton, Chaucer, Dryden, Tennison, Darwin and Livingstone. Monarchs buried include Charles II, William III and Mary, Henry III, Edwards II and III, and Henry V.