According to Play.com Thief of Time
, The Truth
, and Carpe Jugulum
were supposed to be released on June 30th. It seems you can preorder from there.
The Fifth Elephant
seems to be available now.
Personally I disagree with you about Stephen Briggs and think that Tony Robinson is awful at reading the books - but that's just down to personal preferrence. I'd actually prefer to read the books - I can do my own voices then.
The BBC in their shortsightedness never seem to make these plays available to buy. I have also enjoyed all the radio adaptations of the books. But that's the BEEB for you.