BRITAIN'S biggest electricity generator was last night at the centre of a possible £8.5 billion sterling takeover bid by an American utility which could spell further dramatic changes to the electricity sector. A takeover of National Power by Southern Group, which last year bought South Western Electricity - (SWEB) in a £1.1 billion deal, would create a giant super utility involved in generation and distribution. National Power, which commands about 25 per cent of the electricity generation market in England and Wales, is itself trying to buy the regional electricity company Southern Electric.