Maximum,
I understand fully what you're saying, honestly. Maybe I'm not being clear in what I mean.
But the point is that there are two runways available when the decision is made (amongst all the other criteria)
What I'm trying to get across is how do you know that two runways are available? We don't put the fact that cranes are up on the ATIS, because they're up and down all day. The Apron Ops ring us to ask us to raise some cranes, we look on our copies of the permits to see what effect it has, and then say yes or no. If it precludes landing on one runway, we just turn off the appropriate ILS (after telling Radar) and the two air controllers have a bit of paper in front of them saying: "No landings 27L (say) due cranes"
I wasn't saying that I think it's safer to stay at destination buring diversion fuel, just that it's not automatically safer to bank on LHR having another runway always available if one is closed, thus is often
no safer than a single runway airport.