Probably because there is no isolator between the suppliers fuse and the fusebox.Why is the supply to the shower fuse wired into the fuseboard and not taken to a henley block inside the trunking and then to the meter?
Cheaper just to stick it in the fusebox than call out and pay for the DNO to remove the fuse while the work is carried out and refit the fuse after it has been done.
So what's it been wired to inside the fuseboard? The lighting circuit?