A B777 FLT CREW RPTED THE UNWILLINGNESS OF THE EGTT ARTCC CTLR TO VECTOR THE FLT AROUND CUMULO NIMBUS; CAUSING THE CAPT TO DECLARE AN EMER TO AVOID THE WX; WHICH RESULTED IN A VIOLATION BEING FILED.

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DEPARTED EGKK ON THE LAMBOURNE 4M DEP OFF RWY 26L. AFTER TURNING AND AT 5000 FT; WE OBSERVED TSTMS AHEAD AT 5-10 MI. LONDON DEP CTL ISSUED A 090 DEG VECTOR WHICH TOOK US OFF THE DEP COURSE AND ALSO WOULD HAVE TAKEN US THROUGH THE TSTMS. THE CAPT TOLD THE CTLR HE NEEDED A DIFFERENT VECTOR FOR WX SAFETY REASONS. THE CTLR REFUSED AND THE CAPT DECLARED AN EMER DEVIATING TO THE N. THE CTLR STATED THAT HE WAS FILING A RPT AND MADE A NUMBER OF ADDITIONAL COMMENTS NOT RELATED TO NAV WHICH WERE HIGHLY DISTRACTING IN A CRITICAL PHASE OF FLT. SUPPLEMENTAL INFO FROM ACN 624670: DEP FROM LGW ON THE LAM4M AT 5000 FT WITH TSTM ON RADAR. REFUSED HDG THAT WOULD HAVE TAKEN US INTO CTR OF STORM. DECLARED EMER (MAYDAY CONFIRMED BY CTLR). TOLD CTLR WE WERE TURNING TO HDG OF 060 DEGS THEN TO 050 DEGS. CTLR STATED HE WAS RPTING A VIOLATION. SUPPLEMENTAL INFO FROM ACN 624671: ON CLBOUT; CTLR GAVE HDG DIRECTLY TOWARD A LARGE TSTM. CAPT ASKED THE CTLR FOR NEW HDG BUT WAS DENIED. CAPT DECLARED AN EMER AND TURNED TO AVOID THE TSTM. 2 MINS LATER; WE TURNED BACK ON COURSE WITH NO FURTHER INCIDENT.