Cut the bread into thick (1/2 inch) slices, removing the crusts if desired. Spread each slice thinly with butter and cut into quarters (or halves if slices are very small). In a large buttered baking dish (casserole or souffle), arrange one-third of the buttered bread in a layer on the bottom. Sprinkle a half each of the raisins, lemon or orange zest and sugar evenly on top. Arrange another one-third of the bread on top, sprinkle with remaining raisins, zest and sugar. Arrange the remaining bread, buttered side up, on top. In a bowl, lightly beat the eggs and yolks with the milk, cream and rum. Pour this mixture into a spouted jug, for easier pouring. Pour the egg mixture down into the side of the baking dish. Avoid getting the top layer wet or it won't crisp up. (Depending on the volume of the dish, you may not use all the mixture, or you may need to add a little milk so the liquid comes up almost to the top layer. Sprinkle sugar on top. Place the baking dish in a large roasting pan containing a shallow layer of boiling water. Place in a 350F oven and bake for about 30 to 40 minutes, until the custard is set and the top is brown and crusty. Serve hot from the dish accompanied by creme anglaise.