PF FO ON INTL B777 DEPARTING FROM EGLL STARTED INITIAL COURSE CHANGE AFTER TKOF BASED ON THE WRONG SID. ALERT CREW CAUGHT THE ERROR QUICKLY AND FLT REJOINED CLRED RTE.

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MANAGED TO MOMENTARILY TURN ONTO THE WRONG SID DEPARTING EGLL. LOADED A SID FOR RWY 27L AT EGLL ON PREFLT IN ACCORDANCE WITH ATIS; THE RELOADED COMPTON 5J SID FOR RWY 9R WHEN WINDS CHANGED. ASSIGNED BUZAD 3J ON ENG START CLRNC. LOADED THE BUZAD 3J SID INTO THE FMS AFTER ENG START; WHILE SITTING WITH BRAKES PARKED AWAITING TFC. REVIEWED THE BUZAD 3J WITH THE CAPT AND RELIEF FO TO INCLUDE ALTS ON LEGS PAGE OF FMS; AND NOISE ABATEMENT CONSIDERATIONS; ALL BEFORE TAXI. NO PARTICULAR RUSH ON TAXI; NO EXTRAORDINARY EVENTS OCCURRED DURING THIS TIME. MY TKOF. I THOUGHT I HAD OPENED MY DEP PROC FOR THE BUZAD 3J AT THAT POINT. TKOF BRIEFING NORMAL; UNEVENTFUL TAXI AND TKOF; UNTIL REACHING THE 2 DME TURN POINT. AT THAT POINT I MISTAKENLY THOUGHT WE WERE FLYING THE COMPTON 5J; AS THAT WAS THE PAGE OPENED IN THE BOOK; AND AS I REMEMBER THINKING THE PROC THROUGH IN THE EVENT IT WAS ASSIGNED IN THE FLUID RWY SIT; AND BEGAN A TURN TO THE R. FOLLOWING IMMEDIATE INPUT FROM THE OTHER CREW MEMBERS; I REALIZED MY MISTAKE AND CORRECTED; TURNING L IN ACCORDANCE WITH THE BUZAD 3J SID. THE ATTENTIVENESS OF THE CREW AND THEIR UNHESITATING RESPONSE LIMITED DEV FROM THE TURN COURSE TO WHAT APPEARED TO BE ABOUT A 1 MI OVERSHOOT. ATC DID NOT SAY ANYTHING. WISH I COULD EXPLAIN THIS ONE. THE PREFLT AND TAXI OPS WERE WELL PACED; THOUGHTFUL; AND THOROUGH; AND THE CREW WAS PROFESSIONAL AND ATTENTIVE DURING BRIEF. WE LOOKED AT SEVERAL POSSIBILITIES BASED ON WHAT WAS GOING ON WITH THE WINDS AND ON WHAT THE OTHER ACFT WERE BEING ASSIGNED. NOTHING BROKE; MALFUNCTIONED; OR CAUSED ANYTHING OUT OF THE ORDINARY; OTHER THAN A RWY CHANGE. I HATE TO CHALK IT UP TO A BAD DAY; BUT IT CERTAINLY WASN'T A GOOD ONE.