Southern was the subject of a pounds 2.8bn bid from National Power, the nation's largest generator, until it was referred to the Monopolies and Mergers Commission.
It is one of the biggest and most profitable of the regional electricity companies and the new profits made by the directors are bound to fuel Labour Party criticisms of the rewards given to utility company executives.
The gains emerged only days after National Grid announced changes to its directors' share options which could leave them substantially better off than before the flotation of the company in December.
The second-biggest gainer in Southern Electric was John Forbes, who exercised more than 22,000 share options to make a profit of just over pounds 101,000. Jim Deane exercised more than 13,000 options for a profit of pounds 61,000 and Jim Harte just under 13,000 for a profit of pounds 58,000.
All four sold some of their shares to pay tax bills and each transferred a proportion of the remainder to his wife, while keeping the balance. Mr Casley, for example, sold 21,494 and transferred 10,000 shares to his wife.
The decision to refer the bid for Southern by National Power came as a blow to both companies, which had agreed the deal.
The MMC, which is expected to deliver its report to the Government by 22 March, is also investigating an agreed bid for Midlands Electricity by the rival generating company, PowerGen.