LAWSON SAYS U.K. ELECTION NOT ONLY EMS BAR British Chancellor of the Exchequer Nigel Lawson said the next U.K. Election was not the only element standing in the way of full British membership of the European Monetary System (EMS). But he added that arguments against joining had weakened. Prime Minister Margaret Thatcher, widely believed to be the strongest government opponent of full EMS membership, has made clear she does not expect to consider joining until after the next UK election, due by mid-1988 at the latest. But Lawson, in answer to a question, told reporters after an informal European Community finance ministers' meeting here that other factors apart from the upcoming election stood in the way of full membership. In addition to the question of the exchange rates at which Britain should enter the EMS's core exchange rate mechanism, there was also the impact of sterling membership on the system to be considered, he said. British entry would change the EMS from a monopolar system based on the West German mark to a bipolar mark-sterling system, he noted. "We have to make sure it would work." But Lawson added that some of the considerations that had made it difficult for Britain to join in the past now posed less of a problem. As an example he cited sterling's role as a petrocurrency, which he said was diminishing. "That argument has clearly weakened," he said. Lawson restated that the government was keeping the question of membership under review.