(Reuters) -Shares of British power generating firms plunged on Tuesday after a news report that Britain had ordered plans be drawn up for a possible windfall tax on more than 10 billion pounds ($12.6 billion) of excess profits made by the companies. British Finance Minister Rishi Sunak is seeking to raise cash to support households facing higher energy bills, the reported on Monday. Asked for comment, the Treasury did not provide a specific response to the article. Shares of British power generators — Drax, Centrica and SSE — were down between 9% and 18%. They are on track for their worst day …