LONDON -- Standard&Poor's today has assigned its triple-`A' long- term rating to Nippon Telegraph and Telephone Corp.'s (NTT) UK250 million straight bond due Dec. 2, 2003. The rating outlook is negative. The rating reflects NTT's dominant position and important role in the Japanese telecommunications market, and strong cash flow and financial profile among major international peers. Despite increased penetration by the New Common Carriers (NCCs), NTT still maintains over 90% share of the domestic telephone market. NTT's consolidated operating income increased by 53.7% for the fiscal year ended March 31, 1996, as a result of increases in basic monthly charges and directory assistance services implemented in early 1995, and owing to continued growth of the Japanese cellular phone