Preheat the grill to high. Line the rack in the grill pan with foil and spread the oatmeal over the foil. Toast under the grill until the oatmeal is golden, stirring once or twice, about 3 minutes. Set aside to cool.
Put the double cream in a bowl and whisk until thick. Stir in the honey and whisky, then fold in 4 tablespoons of the toasted oatmeal.
Reserve a few whole berries for decoration, and stir the raspberries, blackberries and blueberries together in a bowl. Layer the remaining berry mixture with the whisky cream in 4 glass serving dishes, starting with raspberries and ending with a layer of the whisky cream.
Decorate each cranachan with a sprinkling of the remaining toasted oatmeal and the reserved berries. Serve immediately (or keep in the fridge for up to 1 hour before serving).
Do use a hand whisk to whip double cream. If you go too far, you'll have butter!