ACR WDB ALT DEV OVERSHOT DURING CLB OUT AT LHR. FLC FORGOT TO SET ALTIMETERS TO QNH AT 6000 FT.

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WE WERE CLBING OUT OF LONDON'S HEATHROW ARPT IN A WDB. I WAS THE PF. MY POS IS FO. THERE WAS A CAPT AND RELIEF PLT ABOARD IN THE COCKPIT AT THE TIME. AFTER TKOF NUMEROUS HDG AND ALT CHANGES WERE GIVEN. TURN R TO 285 DEG (AFTER TKOF 9R). CLB TO 6000 FT. THEN ADDITIONAL STEP TYPE CLBS AT THE SAME TIME ADDITIONAL HDG CHANGES. (OUR ALTIMETERS WERE SET TO HECTOPASCAL MILLIBARS). THROUGH 6000 FT; ON OUR WAY TO 8000 FT THE AUTOTHROTTLE SYS PROVED INOPERATIVE RIGHT THEN A NEW HDG CHANGE AND CLB WAS GIVEN TO 10000 FT ALONG WITH SPD OUR DISCRETION. THE GLASS COCKPIT WORKLOAD WAS HIGH SINCE THE CAPT WAS MANUALLY TURNING ALT; HDG AND SPD KNOBS AND I; THE FO; WAS MANUALLY CTLING THE THROTTLES AND FLYING. UPON LEVELING AT 10000 LONDON ATC COMMANDED US TO TURN L AND DSND TO 10000. WE TOLD THEM WE WERE LEVEL. AT THE SAME TIME WE REMEMBERED THAT 6000 FT WAS THE TRANSITION ALT TO 1013 HP/29.92 WA. WE SET OUR ALTIMETERS AND FOUND WE WERE 450 FT HIGHER THAN WE THOUGHT. NO NEAR MISS OCCURRED. WE APOLOGIZED TO LONDON AND THEY SAID THEY WOULD TAKE ACTION. OUR AIRLINE FLYS DOMESTIC INTL AND THE TRANSITION ALT IS A MEMORY ITEM NOT INCLUDED ON ANY OF OUR CHKLISTS.