In a heavy-bottomed pot, melt the butter over medium heat. Once melted, whisk in the flour, and cook until the color shifts slightly darker, about 4 minutes. Add half the stout, making sure the beer is fully incorporated into the roux before adding the rest. Add half the cream, making sure it is fully incorporated into the mixture before adding the rest. Whisk in the mustard and cayenne to incorporate fully with no lumps. Cook until the mixture thickens nicely, 5 to 7 minutes. Be careful not to burn the bottom of the pan. Reduce the heat to medium-low and slowly add the cheddar, whisking after each addition. Be mindful of the heat: if the cheese begins to separate from the roux, cut the flame entirely. Let the rarebit mixture cool completely.
Slice the bread 1¼-inch/3cm thick and toast until golden. Let cool and top each slice with 75 grams of rarebit mixture. Use an offset spatula to spread the mixture evenly over the top of bread. Broil (grill) until bubbling and slightly darkened. Drizzle Worcestershire sauce over the hot rarebit, slice into 5 pieces, and serve immediately.