Wrap a strip of acetate around the outsides of four ramekins and secure with tape. Alternatively use a strip of lightly oiled greaseproof paper.
For the soufflé, reserve 12 raspberries for the garnish and set aside. Place the remaining raspberries into a food processor and blend until smooth. Pass the raspberry purée through a fine sieve into a bowl and set aside.
Place the sugar and water into a pan and gently heat until the sugar has dissolved, then bring to the boil and cook until the temperature of the mixture reaches 115C/239F (check using a digital thermometer).
Meanwhile, whisk the egg whites until soft peaks form when the whisk is removed. Once the sugar has reached the correct temperature, slowly pour the hot sugar syrup into the meringue while whisking continuously. Whisk until all of the sugar has been incorporated and the meringue is smooth and glossy.
Whisk the double cream in a clean bowl until soft peaks form when the whisk is removed. Gently fold the whipped lactose free cream into the raspberry purée, then fold in the meringue until just combined. Spoon the mixture into the prepared ramekins, then place into the freezer until set.