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?

The rubber cable should be regarded as somewhere between at risk and
immediately dangerous. You certainly should not disturb it. It absolutely
MUST be changed as soon as possible. If there are money troubles, there may
be a scheme with the local council to loan money for the work at low
interest, or in return for a charge on the property.

The only modification I would consider would be to install a new consumer
unit and run the entire existing installation from a 32A RCBO using new PVC
cable to the old fuse box entry. Doing this I would NOT touch any rubber
cable at all (except when totally removing a section). I would also regard
this as an emergency fix until an electrician can come to rewire the entire
property. I would only do it if the rubber appears to be in good condition
and I could trust the residents to be sensible (i.e. they are family, no
children live in the house, and they are not members of the public).

You may well find that the RCBO constantly trips. If so, the cable is
immediately dangerous, rather than simply at risk. Entirely disconnect the
electrical installation and find temporary accommodation for your father.

Christian.