Britain proposed cuts in the price of its North Sea oil - but they were limited. British National Oil Corporation, the state trading company, proposed setting the price of its marker crude at $30 a barrel, 50 cents down from a price recommended last month. Other grades would drop 75 cents a barrel. Britain is caught in a balancing act, trying to satisfy its customers and at the same time avert a price war with its competitor, Nigeria. Some observers have predicted such a price war if Britain's price fell below $30 a barrel.