How to detach box from skirting board
I would aim to pry out the top to see how the box is fixed in place, then disassemble the box and, if necessary, rebuild a new one from new wood.

Even replacing cut/damaged 240VAC wires is not an insurmountable obstacle.

Deciding that you are fully prepared to deal with the worst possible outcome is, for me, an important step in raising the necessary motivation to start on a difficult job.

Turn off the main electrical panel first. Take whatever other precautions seem appropriate.

There's a limit to how much damage you can do here. So long as you take some basic precautions. The sky won't fall.