Beat the butter and sugar together in a mixing bowl until pale and creamy. Gradually mix in alternate spoonfuls of beaten egg and flour until all has been added and the mixture is smooth. Stir in the baking powder, poppy seeds, lemon rind and 5–6 tablespoons lemon juice to make a soft dropping consistency.
Spoon the mixture into a greased 1 kg (2 lb) loaf tin, its base and 2 long sides also lined with oiled greaseproof paper. Spread the surface level and bake in a preheated oven, 160°C (325°F), Gas Mark 3, for 1 hour–1 hour 10 minutes until well risen, the top is cracked and golden and a skewer inserted into the centre comes out clean.
Leave to cool in the tin for 10 minutes then loosen the edges and lift out of the tin using the lining paper. Transfer to a wire rack, peel off the lining paper and leave to cool.
Sift the icing sugar into a bowl then gradually mix in enough of the lemon juice to make a smooth coating icing. Drizzle over the top of the cake in random squiggly lines. Add strips of peel to the top and leave to set. Store in an airtight tin for up to 1 week.