WITH Hong Kong's broker-to-broker system now stretched to the limits, there will be plenty of relief when automated trading starts on November 1. As overseas money poured into Hong Kong blue chips on Monday, under-pressure brokers had to be given an extra 15 minutes to clear a backlog of orders. After more than two years of problem-plagued development, the new $100 million custom-built trading system will be able to handle 200,000 deals - nearly triple Monday's record of 73,792. Stock Exchange chief executive Paul Chow said the new system would be able to handle significantly more trades. ''One hundred thousand deals is nothing,'' he said. During a recent rehearsal, 175,000 deals had been handled easily.