THE patron saint of cigars, David Tang, is impossible. There he was at the launching of Viscount Linley's designer cigar cabinets trying to persuade tycoon Allan Zeman, who doesn't even smoke, to join him in purchasing two so that Tang could negotiate a discount - and with the House of Windsor's finances the way they are, too. Not the sort of form we expect from someone who - as is rumoured in Tang's case - is expected to get a tap on the shoulder with a ceremonial sword from David Linley's aunt at some future date. Any more of this sort of thing and the ceremony might well be moved from Buckingham Palace to the Tower of London. And the sword will wield not a gentle tap but a mighty chop. The presence of the Queen's nephew obviously affected one Brit present, a long-time resident in Hong Kong, in an emotional sort of way. With the guilt of living in self-imposed exile from Old Blighty probably heavy on his patriotic bosom, he made amends to Queen and country by buying two of the cabinets (prices ranging from $20,000 to $120,000) - and he doesn't like cigars either. Seeing a socialite admiring one of the boxes, we suggested to her that it could also be used to store jewellery. But it was only when one of the organisers mentioned that Linley would probably autograph it for her that her cheque book came out. Incidentally, spotting one of this town's most distinguished Englishmen engaged in earnest discussions with the Royal guest of honour, Keeping Posted later inquired of him whether the matter of a ''K'' had by any chance been dropped into the conversation. ''Knighthood be blowed'', he growled. ''I'll settle for nothing less than a barony, dear boy . . .'' Arise, Lord Rodway of Shamshuipo.