Determining in-flight weather conditions is the pilot's responsibility, and under a SVFR clearance, helicopters must remain clear of cloud and in sight of the surface. Additionally, a pilot must remain in flight conditions that enable him to determine his flight path, remain clear of obstacles, and operate in accordance with Rule 5.
Battersea will normally be closed to traffic unless the cloudbase is 600ft or greater, and/or the met vis is 1000m or greater.