BRITISH POLITICIANS URGE JAPAN TRADE SANCTIONS One hundred members of Britain's ruling Conservative Party have signed a motion calling for trade sanctions against Japan to force Tokyo to open its domestic market to British goods. The government announced last week that Japan had a 5.9 billion dlr trade surplus with Britain in 1986. The Department of Trade and Industry said the government was drawing up contingency plans to force Japan into opening up its domestic markets but a spokesman said such moves were very much a last resort. Ideas being considered included blocking Japanese companies from trading in Britain and revoking licenses of Japanese operations in the London financial district.