Anti-icing and de-icing are different. Anti-icing is done to protect the aircraft from further precipitation while de-icing is done to rid and aircraft of precipitation that has already happened. It can be that the two processes happen at the same time, which is what tends to happen at LHR on frosty mornings. The aircraft are de--iced and anti-iced in the same process which removes the frost and prevents further frost from forming. In heavy on-going precipitation it tends to be a two step process and is often done just before takeoff, often at a remote place close to the runway.