Heavy cream has the highest fat content of any cream in the U.S. dairy market, so in that sense, it is the equivalent of double cream, but I believe that double cream actually has a somewhat higher fat content, but still close enough to that of heavy cream that they can be used interchangeably in recipes.
The butterfat content of milk varies considerably from cow to cow. Some breeds give milk with more butterfat (such as Holsteins, as I recall), whereas breeds primarily intended for the table (Angus, for example), typically give milk with less butterfat. In the U.S., commercial dairies pool the milk from many cows to standardise the fat content.