The announcement of a substantial impairment charge is a negative signal to investors, reflecting potential overvaluation of assets or recognition of losses. This can lead to a loss of investor confidence and a decrease in stock price, as evidenced by the 9.6% drop. The magnitude of the charge (£25 billion) is significant, which justifies the high importance and relevance scores. The confidence score is high due to the clear cause-and-effect relationship between the impairment charge announcement and the stock price movement.