Cook the rice in boiling water until nearly done. When there is about 5 minutes to go, make the prawns.
In a large frypan, combine butter, cayenne, pepper, salt, chilli flakes, thyme, garlic and Worcestershire sauce over high heat. When butter has melted, add the prawns. Cook 2 minutes, shaking the frypan so the butter doesn't separate.
Add stock and cook, shaking, 2 more minutes. Add beer and cook 1 more minute. Serve immediately with a mound of white rice and the sauce poured around.