There wouldn't be workers in the tunnels if trains were running, so no problem there.
The only exception being something which can be quickly remedied, in which case the engineers will travel to the site onboard a train, then use it as protection. A good explanation here.
Obviously, in this case though there is no need for the workers to be able to see the train from a distance. You also have to remember that the maximum speed on the LUL network is much lower than on NR, therefore workers on tracks outside the tunnels have more time to react to an approaching train.