Line a 30 × 20 cm (12 × 8 inch) Swiss roll tin with baking paper. Whip the egg whites until stiff peaks appear, then add the sugar, a tablespoon at a time, and keep whisking until you have a stiff, glossy meringue. Stir through the vanilla extract.
Spoon the mixture into the tin and smooth over the surface. Place in a preheated oven, 200°C (400°F), Gas Mark 6, and immediately reduce the temperature to 160°C (325°F), Gas Mark 3. Cook for 15 minutes until firm to the touch.
Turn the meringue out on to a sheet of baking paper dusted with icing sugar and leave to cool for 3 minutes. Peel off the lining paper from the top of the meringue, then carefully roll up the meringue from the long side. Set aside to cool.
Whip the double cream with the icing sugar, unroll the meringue and spread the cream on top. Mix the jam with a splash of boiling water to loosen, then spoon over the cream and top with the sliced strawberries. Using the baking paper, roll up the meringue and place on a serving plate. Dust with some more icing sugar and serve.