Don't know if she speaks like that in real life, but in the context of the story she's a well to do Vicar's daughter & it's a pretty standard stab at an upper middle class British accent much favoured by films of the period for the RAF Pilots and commissioned army officer types. As opposed to the 'lor luv a duck' ** chirpy cockney aircrew or conscripted private types. ** or North East Geordie or rural yokel stereotypes too.
if you want to practice with more of the same, try Call The Midwife - semi autobiographical Nurse/Midwife team set in the very impoverished east end of London. The Nurses are all quite plummy voiced (as Sam) while the residents that they're treating are very much of the common clay.