The council's environmental department, and tell them that the cat is laying on the pavement or road (council-public owned property), they are then legally obliged to remove the deceased item. On private property, it is then the property owners responsibility.
Normally they then would 'refrigerate' the item for 7 days in case the owner turns up and reclaims the body, the same procedure applies to dogs and similar 'pets'.
Depending on how strict your council, then it can be bagged and placed in the wheelie bin, for disposal on the next collection day. If its 'maggot' infested, then spray the cat with fly killer or something similar, because the maggots with most likely escape and make havoc with the other contents of the bin.