Brian Sewell's accent may be genuine, but for me it is too dated.
It is diificult to tell with actors whether the accent they use is their own. If I had to choose between listening to Gielgud or Olivier, I think I'd choose Gielgud. If I had free choice, I'd go for Richard Burton. My choice is probably based on a subjective view of the sound of the voice rather than of the accent.

