Thickened cream and Double Cream contain the same 35% milk fat and are essentially the same thing and the only real difference between thickened cream and double cream is in the name.
In the United States it's sometimes referred to as double cream, heavy cream, whipping cream, and mostly in other countries such as Australia that refer to it as double cream, or thickened cream.
So if you need to use either double cream or thickened cream and can only get one of the two in the United States or where you live then either double cream or thickened cream will work because they're the same thing but with different names.