I am at present going for NICEIC registration and am fitting a consumer unit, the existing old (red/black) wiring is too short, is there a connection box for adding longer wires or can I put the 100 amp isolator in a seperate box outside the original consumer unit
It doesn't matter that you are 'going for Niceic registration' you are breaking the law by altering con.unit wiring, you must be Partp registered before doing this work.
Having said that,
If you mean to fit seperate isolator switch in an enclosure and add extra 'tails' or to use a double 'Henley block' to extend 'tails' either way you would be illegally either breaking cut out fuse seal to pull fuse or doing it 'live' which is more dangerous than people realise and against the law. take care,