So what you're saying is that they can issue 10% more shares at short notice without the need for a prospectus and other possible legal hurdles because they are allowed to issue as many again as they have in treasury (upto to a max of 10% in treasury).
Does this imply that they could have initiated the buyback as an unorthodox way of setting up an issue of more shares? The loss they make if the bought-in shares fall a bit is more than made up for by the proceeds of the issue if it goes off as planned. E.g. buy 10% @ average of, say, 360p then issue at, say, 320. That's a loss of 40p x 10%(of shares in issue) against a gain from the new batch of 320p x 10%(of shares in issue). Maybe they thought the shares would keep rising and they wouldn't lose anything at all on the bought-in batch, nor would they have to pay underwriting costs if they simply placed the shares with institutions by pre-agreement.