Double cream is heavy cream with a butterfat content of 42%. This is hard to find in the U.S. You can substitute "heavy cream" which has at least 36% butterfat. Avoid ultra pasteurized versions which are more difficult to whip and don't have a clean, fresh, cream flavor.
Double cream is used with fresh berries, atop scones or even with tea.
If you don't have (or can't find) double cream, there are some very good options: