B777 CAPT BECAME ILL AND DIRECTED THE RELIEF FO TO OCCUPY THE L SEAT.

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LAX-LHR. CREW: I (CAPT); 2 FO'S; AND 1 FLT ATTENDANT. DURING THE FLT THE CAPT INDICATED THAT HE WAS 'UNCOMFORTABLE.' HE APPEARED TO BE 'ANXIOUS' AND APPREHENSIVE ABOUT THE XING TO LHR. I ATTRIBUTED THIS TO HIS UNFAMILIARITY WITH LAX-LHR RTE. PRIOR TO THE APCH INTO LHR; THE CAPT SAID HE WAS NOT FEELING WELL AND THAT I SHOULD FLY AND LAND THE ACFT AND HE WOULD MANAGE THE RADIO COM (VERY NORMAL PROC). SOON HE WAS FEELING WORSE AND DECIDED IT WOULD BE BEST IF THE (AUGMENT) #2 FO SAT IN THE CAPT'S SEAT. THE CAPT RETAINED COMMAND AND ASSISTED WITH TFC WATCH AS THE 'OBSERVER.' A MESSAGE WAS SENT TO THE LHR OPS EXPLAINING OUR SIT AND TO PLEASE ADVISE ALL CTLING AGENCIES. WE WERE GIVEN EXPEDITIOUS HANDLING AND LANDED UNEVENTFULLY ON RWY 27R AT LONDON. PARAMEDICS MET THE ACFT AND THE CAPT; WHO WAS AMBULATORY; WENT TO A LCL HOSPITAL. WE LATER HEARD THAT HE MAY HAVE HAD A MINOR STROKE. APPROPRIATE COMPANY CONTACT AND PROCS WERE ACCOMPLISHED BY OPS AND FLC POST LNDG. SUPPLEMENTAL INFO FROM ACN 468981: CAPT INDICATED TO BOTH CREW MEMBERS THAT HE WAS NOT FEELING WELL AND WAS CONFUSED AND DISORIENTED. A DISCUSSION ABOUT ALTERNATIVES AND POSSIBILITIES IN CASE OF INCAPACITATION. A DECISION BY ALL CREW MEMBERS WAS MADE. THE CAPT MOVED TO THE JUMP SEAT; THE FLYING FO CONTINUED IN HIS POS FOR THE LNDG; AND ME; AS THE AUGMENTED FO; SAT ON THE L (CAPT'S) SEAT FOR THE LNDG. LONDON OPS WAS ADVISED VIA ACARS OF THE SIT. THE CAPT WAS STILL IN COMMAND EXCEPT FOR THE MANIPULATION OF THE CTLS. CLR AND SOP RESOURCES WERE USED TO CONTINUE THE LNDG AND TAXI TO THE GATE. NO SAFETY WAS JEOPARDIZED AND THE FLT ENDED UNEVENTFULLY. CAPT WAS MET BY AMBULANCE TEAM AND TAKEN TO AN AREA HOSPITAL WHERE LATER WAS DIAGNOSED WITH A MILD STROKE.