Jerricho - your memory serves you well, I think we are talking of the same incident. A level bust at any of Heathrows stacks could be potentially serious if the controller does not try to use some vectors - especially as the SIDs of Heathrow and surrounding airports all interact and often route under the holding areas.
Obviously you would have to take any instance on its own merit, but at the onset, moving aircraft that are above/below the hold as a precaution would seem to be a sensible way of limiting multiple TCAS events.