Just test it right now, cannot replicate it. BAW 777W official livery from PMDG, accents are definitely British. Be sure you set the correct airline code (BAW) in Simbrief, because that's were the airline nationality is read from and it has an higher priority over the one in the airplane livery (so you could fix it if it's wrong or missing).
If you posted your log, I could tell you exactly the problem. Here's the lines to look for, taken from my log, which shows the airline was read correctly and the voices has been set correctly as well:
USER airplane title reported by Simconnect is PMDG 777-300ER British Airways (G-STBC | 2010)
ATC_ID read from aircraft.cfg is G-STBC
ICAO Airline is BAW
Airline Language is ('en-gb', 'euro')
creating SimBrief object
True LSZH LEMD LSZH
icao_airline BAW
flight_number 1115
route VEBI4S VEBIT T50 ROTOS Z669 ULMES DCT VADAR DCT NINTU UN869 REPSI DCT LERGA DCT MINSO DCT NARAK DCT AGN DCT MAQAB DCT TIVLI N869 ZAR Z245 TERSA TERS3Z
passengers (370/370)
airplane B77W
Loading crew sounds from C:\Program Files (x86)\Addon Manager\couatl64\..\Couatl\GSX\sounds\en-gb, time = evening
