Good eye, J-F! Lady Doreen Brabourne's tiara was auctioned at Christie's London in June 2002 (38 240 GBP). The current seller's advertisement "Edwardian diamond crown once owned by English royalty" is a bit of a stretch for both, the provenance and the era – the mother-in-law of Prince Philip's cousin was hardly royalty and Christie's dated the tiara circa 1925 – but it's an impressive piece, even more so when worn, as the photo you posted proves. The photos below show that the central panel is different, so it was either replaced – possibly because of the large solitaire – or the tiaras are of a very similar design.
Christie's