Crush the digestive biscuits into fine crumbs using a food processor or rolling pin. Mix with melted butter, caster sugar, and sea salt until the mixture resembles wet sand.
Firmly press the biscuit mixture into a 20cm springform tin using the back of a spoon or a flat glass. Refrigerate for 30 minutes until firm.
Beat the room-temperature cream cheese with the sifted icing sugar until smooth. Mix in the vanilla extract and lemon zest.
Whip the cold double cream until soft peaks form, then gently fold it into the cream cheese mixture using a spatula until light and fluffy.
Spread the filling evenly over the chilled biscuit base and smooth the top with a palette knife.
Cover loosely with cling film and refrigerate for at least 6 hours, preferably overnight.
Remove from the springform tin and decorate with fresh berries, whipped cream, white chocolate shavings, berry coulis, or a light dusting of icing sugar before serving.