Two reasons he didn't announce his 'prediction' before the draw:
1/ Everyone would have bought tickets with those numbers and won
2/ It is not possible
I like Derren Brown because he does not (unlike many mentalists) claim any special or supernatural powers. He is, in fact, a 'magician' in the sense that David Copperfield et al are magicians - ie he is an illusionist.
His gift, however, is in debunking the nonsense peddled by mind readers, fortune tellers, clairvoyants, astrologers, mediums, palmists and the like. All these types of people are 'magicians' (illusionists) of one kind or another and he shows the malicious and confidence tricksters up for what they are - people who prey on the gullible and/or vulnerable.
I, for one, can't wait to see how he did it - I am sure that nobody will get rich using this method of 'prediction', but I bet it's interesting and innovative.
I 'predict' it is a version of the one-ahead principle used extensively in mentalism.