Hmm - the time when you could wander into a pub in london and hear decentt music has long past :(
Your best bets (now the Intrepid Fox is gone) are
Tiny foxhole of a bar near Foyles - they could be playing anything from Classic Rock to Black Metal on the Jukebox.
You have a better chance of something listenable in Camden. Try the Purple Turtle (Mornington Crescent) or the Dublin Castle, Worlds End, Devonshire Arms (Goffs only).
For full on metal from the depths of Hell, try
"The Marlborough Head", North Audley Street, London (near Selfridges, London)
membership of the Church of Satan not obligatory ... but possibly comes in handy