The originals will net him and his source a nice long term in a British jail for violating the British Secrets Acts.
If the memos succeeded in triggering an attempt to unseat Blair I could envision him being offered immunity in exchange for producing the originals as evidence.
Without them he would be in a lose-lose: either the copies would be ruled unsubstantial and his story discredited or they would be upheld and he would be subject to the penalties you mention, since there would be no need to grant him immunity for what he's already released. He shouldn't be able to have it both ways, i.e. able to claim they are real enough to indict Blair and Bush but not real enough to land him in jail.