Chilled prawn and cucumber soup

Name: Chilled prawn and cucumber soup
Prep time: 10 min
Cook time: 10 min
Total time: 20 min
Cuisine: British
Ingredients: 25 g butter; 1 small onion (finely chopped); 1 clove garlic (crushed); 1 small cucumber (peeled and finely diced); 300 ml milk; 1 tbsp mint (dill and chives chopped); 50 g prawns (cooked peeled and chopped); 300 ml double cream (thick cream); salt and pepper; 6 cooked prawns peeled but with tails left on and ribbons of cucumber to decorate each glass