First, prepare the jelly for the cream: In a bowl, dissolve the watermelon jelly in the hot water, stirring well. Set aside to cool completely.
For the base: Crush the digestive biscuits into a fine powder using a blender. Add the sugar and softened butter, mixing well. Line the base of a 24 cm round springform pan with non-stick greaseproof paper, then press the cookie mixture evenly into the base using the bottom of a glass or the back of a spoon. Refrigerate the pan to set the base.
For the cream: In a mixing bowl, beat the cream cheese, sugar, and vanilla extract until smooth and frothy. Gradually add the cream, continuing to beat until the mixture is fluffy and mousse-like. Reduce the mixer speed and add the lemon juice and cooled watermelon jelly. Pour the cream mixture over the cookie base and refrigerate for 1-1 ½ hours to set.
For the topping: Dissolve the watermelon jelly in the hot water, stirring well. Set aside to cool completely. Once the cream layer has set, gently pour the cooled jelly over the cheesecake. To prevent the jelly from disrupting the cream layer, pour it over the back of a spoon to distribute it evenly. Refrigerate the cheesecake until the jelly has set.
To unmold, run a knife around the edge of the jelly to loosen it from the sides of the pan. Decorate with whipped cream, mint leaves, and blueberries, if desired, before serving.
Note: This cheesecake can be made with other flavors such as strawberry, cherry, or plum.