Crush the shortbread biscuits until they become fine crumbs. Gently melt the unsalted butter and mix it into the crushed biscuits until well combined. Press this crumb mixture firmly into the base of an 8-inch (20cm) deep springform tin to form an even layer.
In a large bowl or the bowl of a stand mixer, beat together the cream cheese and icing sugar until completely smooth. Gradually pour in the double cream while whisking continuously until the mixture starts to thicken.
Slowly add the lemon juice to the mixture and continue whisking until it becomes very thick and creamy. Check the thickness regularly and gently fold the mixture with a spatula to maintain an even texture.
Spread the thickened cheesecake mixture evenly over the biscuit base. Chill the cheesecake in the refrigerator for at least 5-6 hours to let it set fully, or ideally leave it overnight.
For the topping, whisk together the double cream and icing sugar until you achieve a pipeable consistency. Decorate the set cheesecake by piping the cream on top using your preferred nozzle.
Garnish each slice with a lemon slice and a sprinkle of fresh lemon zest before serving.
