When two females have sex with a double ended dildo while it is lit on fire.
Or, for a variation (if possible):
Two females have sex with a crumpet lit on fire, and then pour hot tea on eachother.
Oh man, those chicks did the london broiler and now they're in the hospital!
A varience of the London Fog Horn only the recipient is made aware of the act before hand as to prepare by having a lighter handy. When the manufacturer of the London Fog Horn preforms his task (farts in her mouth)the woman keeps the gas in her mouth only to light the lighter in front of her face and exhale the flamible substance onto the lighter, which creates a flame of biblical proportions.
Man, when that bitch did the "London Broiler" last night, she stunk up my room with the smell of my own burnt ass hair!