You might want to contact the RSPCA since swans are a protected species. If they do not pick up the carcass, they should be able to tell you who to contact.

Secondly, Mute Swans and other species do work together for a common goal. Mute Swans can go upside down and reach sub-aquatic vegetation. These types of macrophytes house fish larvae and other plankton that live among the vegetation. Because swans have such long necks, they can reach these food sources that other waterfowl, i.e., ducks, herons, egrets, etc., cannot.

The swans feed by pulling at the sub-aquatic vegetation and bring the food source to the top or just below the water level. The other birds can then feed along with the swans. This might very well be the reason that the Heron was staying around the Mute Swan as it was dislodging hard to reach food resources. The Regal Swan