FROM 18 July, Advanced Channel Express (0273 818220) is operating a passenger service from Brighton Marina to the Normandy resorts of Dieppe and Fecamp, using a 200-seater high-speed catamaran. The crossing is scheduled to take two hours. Day-trip return fares: pounds 20 midweek, pounds 25 weekends; normal unrestricted return costs pounds 60. The service will run for two months this year; in 1993, the company hopes to operate the Channel Express for a seven-month period.