My Father's asked me to replace his fusebox for a slit load CU, he
doesn't want to rewire the house which I have suggested as its rubber
cable. I havent looked at the job propery yet but my question is, what
to do if the cables are too short for the new places in the new CU?

I would prefer to at least fit the CU as it will offer more protection
than they have now, which basically is nowt !! If I can I'll pressure
them into a rewire.