Technically illegal?

[speedking31]

Near me are some major roadworks currently involving a significant amount of earthmoving plant. The works are on both sides of an A road. A light controlled crossing has been established. The red traffic lights are used to stop main road traffic and then several large dump trucks cross the road. As these vehicles are not registered, are they breaking the law by driving on a road, even though they only cross it at right angles?

Not bothered by this, just interested if there is some sort of exemption?