1) Peel and pit the avocados. Add the avocados, lemon juice, milk, and sugar to a blender and puree. Transfer the mixture to a medium mixing bowl, add the double cream and whisk to combine. Place the mixture into the refrigerator and chill until it reaches 4C or below, approximately 4 to 6 hrs.
2) Process the mixture in an ice cream maker according to manufacturer's directions. However, this mixture sets up very fast, so count on it taking only 5 to 10 mins to process. For soft ice cream, serve immediately. If desired, place in freezer for 3 to 4 hrs for firmer texture.