There is some ambiguity regarding the sourcing. While OBJ does claim that the Pakistani source is credible, one can't expect people (especially Indian apologists) to just accept that 'credibility' without question. I would agree with the Indians here, and were this an Indian allegation against Pakistan, I would take the same position they are.
Now, what adds another dimension to these allegations is that the 'credible Pakistani source' quoted some specifics related to these 'confessions' being made to UK law enforcement. I would hope that OBJ obtained unofficial/ off the record confirmation of said claims from the relevant UK authorities before going public with this story, which would make these claims much more credible. That said, if the confirmation from UK authorities to OBJ was off the record, that means the UK government will not come out publicly with confirmation, especially given the fact that the Modi led BJP government will throw the world's biggest temper tantrum and whine-fest. So the audience then has to rely on the history and credibility of the journalist claiming he obtained corroboration from UK officials.
I would like to wait and see if OBJ says he corroborated the claims, by Pakistani officials, from UK government/LEA sources.