INBOUND TO EGLL; DUE TO WX AND INFREQUENT EXPOSURE; B777 DOES NOT RESET TRANSITION ALT ALTIMETER; CAUSING AN ALT OVERSHOOT AND ALERT BY EGTT CTLR.

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HEATHROW DIRECTOR ISSUED A CLRNC TO 6000 FT DURING THE DSCNT FOR APCH TO RWY 27L. THE FO WAS LOOKING IN CHARTS AT THE APCH AND THE PLT WAS LOOKING UP THE FREQ FOR OPS TO GET A GATE ASSIGNMENT. THE CAPT LEVELED AT FL060 AND THEN RESET HIS ALTIMETER FOR THE TRANSITION ALT OF 6000 FT. DUE TO THE LOW ALTIMETER SETTING OF 997 MILLIBARS (49.43 INCHES); THE ACFT WAS APPROX 460 FT LOW. THE CTLR ISSUED AN IMMEDIATE CLB TO 6000 FT AND WE COMPLIED. WE WERE NOT AWARE OF ANY IMMEDIATE TFC CONFLICTS. BOTH THE CAPT AND FO HAD NOT BEEN TO LHR IN A WHILE AND UNDERSTANDING THE BRITISH CTLRS WAS ALSO A PROB. SUPPLEMENTAL INFO FROM ACN 507551: I DID NOT RESET MY ALTIMETER TO LCL ALTIMETER SETTING. CREW WAS EXTREMELY TIRED AFTER LONG NIGHT ON DUTY; COMPOUNDED BY LENGTHY 2 HR DELAY AT GATE IN THE UNITED STATES; COMPOUNDED BY UNBELIEVABLE NUMBER OF ALT; SPD AND HDG CHANGES BY CTLRS WHO WERE DIFFICULT TO UNDERSTAND DUE TO ACCENTS; HURRIED ENUNCIATION AND DROPPING VOWELS AT THE END OF WORDS. SUPPLEMENTAL INFO FROM ACN 507552: BECAUSE IT WAS SO CLOSE TO 29.92 WE DIDN'T REALIZE HOW LOW THE ALTIMETER WAS. APCHING THE TRANSITION ALT OF FL060 I PUSHED THE ALTIMETER BUTTON AND REALIZED WE WERE ALREADY AT 5600 FT. I CALLED OUT TO THE CAPT THE ALT AND AT THE SAME TIME TWR CALLED OUR ALT. THERE WAS NO TFC CLOSE TO US AND WE CONTINUED AND LANDED UNEVENTFULLY.