<<I'm not going into the criteria because they're in the TC MATS Pt 2 and it's not my place to publish stuff from that here, but suffice to say it involves final approach and parallel runways.>>
Roffa.... Yes, but 2nm is not a radar separation surely? Are you talking about visual separation between aircraft on parallel runways? In which case there is no defined separation.
I shall be seeing one of the Heathrow Directors this morning and will be finding out more!! 2.5nm separation on final approach was the minimum when I was there, but that's long ago!