Preheat a grill or grill pan over medium-high heat.
For the syllabub: Combine the sugar, sherry, brandy, vanilla and lemon juice and zest in a small bowl and stir to dissolve. Start beating the cream with a hand whisk or an electric mixer in a large chilled bowl. Gradually add the brandy and sherry mixture, whisking constantly, until the cream is fluffy with soft peaks. Keep cold while you prepare the pound cake.
Brush both sides of the pound cake slices with the butter and sprinkle with the sugar. Grill, uncovered, over indirect medium heat until light golden brown, 1 to 2 minutes per side. Serve immediately, topped with the berries, syllabub and chopped basil, if desired.