I made a tart-type-thing once that was really good... I used store-bought pie crust made from shortbread cookies, but you could probably find/make something lower in calories... then put in a mixture of equal parts cream cheese and cool whip (both low fat)... and top with your favorite sliced fruits, and a sprinkle of sugar. Keep it refrigerated right up until serving, the warmer it gets the harder it is to slice.
If there's no equivalent of cool whip in the UK, maybe you could use a low-fat whipped cream instead... you might need to add some sugar or sweetener, though, cool whip is ungodly sweet.