Prices dipped in dull trading on the London Stock Exchange Friday, hurt in part by weakness on Wall Street. The Financial Times-Stock Exchange 100-stock index closed down 6.3 points, or 0.4 percent, at 1,729.8. Volume was a low 377.8 million shares, compared with 411.3 million Thursday. Prices were marked down at the opening in response to a fall Thursday in New York, brokers said. Prices barely moved for the next several hours because of a lack of trading and interest, they said. But prices slipped again in the afternoon when Wall Street failed to revive at its opening. The market currently is lacking direction because it is receiving contradictory signals about the direction of the economy, and is waiting for British companies to complete reporting their 1987 results, one broker said. Only a few companies have already reported their results, and the market needs some indication of how Britain's major banks and industrial concerns are doing, the broker said.