To substitute double cream (heavy cream) in a recipe with whole milk, you can melt 1/4 cup of butter and whisk it with 3/4 cup of whole milk for every cup of double cream required. 
Here's a more detailed explanation:
Understanding the Difference:
Double cream (heavy cream) has a higher fat content than whole milk. 
The Substitution:
Melt 1/4 cup (or 50g) of unsalted butter. 
Whisk in 3/4 cup (or 180ml) of whole milk until smooth. 
This mixture will equal 1 cup (or 250ml) of double cream. 
Why this works:
The butter adds the necessary fat content that double cream provides, while the milk helps to create the desired consistency. 
Other Considerations:
This substitution works well in most recipes, but may slightly alter the texture of baked goods. 
If you're whipping the cream, the substitution may not whip as well as double cream, so be mindful of the recipe and desired result.