Double cream is the name in Britain for a very rich cream - containing 48% butterfat. Whipping cream in this country, by contrast, contains between 30% and 40% butterfat. Single cream in Britain is comparable to American half and half (and may also be called pouring cream), with between 10% and 12% fat.
Double cream is so rich, in fact, that it is easy to overwhip it and get it too thick. Some cooks add a tablespoon or so of milk to 8 to 10 ounces of double cream before whipping it to keep it loose enough.