By Sarah Mills LONDON (Reuters) – Royal portraits and dazzling tiaras go on display at Sotheby’s in London this weekend as part of celebrations to mark Queen Elizabeth’s Platinum Jubilee. The auction house is holding a month-long programme of events, exhibitions and auctions to mark the occasion. Four days of official celebrations for the historic occasion begin on June 2. At Sotheby’s, artworks including the Armada Portrait of Elizabeth I, commemorating the failed invasion of England by the Spanish Armada in 1588, and Andy Warhol’s 1985 painting of Elizabeth II, will be on show alongside depi…