The lane is shut to protect the people working on the roadside. It would have been coned off for a set distance before the area that they planned to work in, the planned work area itself and then a set distance after the work area. There are a lot of road workers seriously injured or even killed each year due to drivers not realising they are there or not giving them room to work so now-a-days they cone off the entire lane if necessary to protect them.
As for the M25 we never touch that road without checking the
BBC travel,
AA Travel and
Highways Agency sites to see what roadworks, accidents etc are on-going as there are a lot of late night engineering projects ongoing at the moment.