Ronald Reagan may have a few tips on retirement to share with his friend Margaret Thatcher, who recently was forced out as British prime minister. Reagan, who has filled his post-presidential years with big-money speaking engagements and writing memoirs, and his wife, Nancy, are in London with plans to visit Thatcher, Prince Charles and Princess Diana and Queen Elizabeth and her mother, the queen mum. Reagan's schedule also includes a speech to Cambridge University's Cambridge Union Society, a dinner talk at the Atlantic Richfield oil company and an appearance before a British charity fund-raising club before returning to California Saturday. Sir Ronald, as he can be addressed in Britain since receiving an honorary knighthood from the queen in 1989, also was on a television talk show.