Ann: Change of Directors Interest Notices, page-6

It's not that directors aren't allowed to sell ever.  But they do have insider knowledge.  The know what's coming down the pike long before that info is crystalized and released to the market. Now, if a single director sells his shares, that may be meaningless.  If several all sell their shares at the same times, some investors are naturally suspicious.

Directors get dividends just like you and I.  Plus director's fees.  So they're paid well already. It can be seen as a loss of confidence in the company if a director sells.  I guess the "right time" for a director to sell is when he ceases to be a director, from purely an optics POV.