I'm trying to work out if that loop is broken and no longer connected to the L ( this is the permanent live) . As other have said please make sure the power is turned off before you touch anything
if the neutral and earth of the flex are touching together they will trip the rcd out even with the fuse removed from the plug . But what you are doing does not sound safe.
Edit
Westward beat me to it :)
Clamping a peace of wood inside. Before drilling may help prevent dislodging the knockout . I have not use fuse box so dont know how secure the knockouts are.
If you have spare normal open contact you can send a 24v contactor coil supply through the normally open contact Then you will need to fit a push to break switch on the same circuit to reset the contactor
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