Cleared only to the transition altitude departing EGLL; B767-300 flight crew set QNE prematurely and leveled off nearly 800 feet low due to the low altimeter setting.

Narrative

On takeoff from Runway 9R LHR; MUZAD 3J Departure; I mistakenly set altimeter to QNE when approaching the transition altitude (6;000 FT) instead of when climbing through the transition altitude. Since the altimeter was so low (987 MB or 29.15 inches Hg) and the first altitude restriction was 6;000 FT; I flew around for 5 minutes or so at an altitude of 5;230 FT. LHR Departure Control never questioned me or asked me to confirm my altitude; so I did not realize that I was at the wrong altitude. It was not till after we leveled off at our cruise altitude and looked it up did I realize that I erred. I think that contributing factors could be: 1) This is different from the way we do it in the United States; i.e.; we set QNE when 'at or above' the transition altitude; and 2) The only place that I could find guidance about this is in a training manual. Maybe you could consider putting this information on the LHR briefing pages. There is a note on the LHR briefing page. 'The initial leveloff on departure is critical with London ATC. Verify the appropriate altimeter setting (QNH or QNE) is used for the altitude/flight level assigned.' The confusion was not the value of the altimeter setting but WHEN to set the altimeter to QNE. Perhaps cutting and pasting the information from this training manual to the LHR briefing pages would help.