A couple of points.
You say the CU is in a metal cupboard, is this a skeleton board, where the metal cupboard is the outer case of the CU?
If it is, then I suggest doing a search for skeleton board, and you should be able to find some info.
Yes you can have a CU wth a single 30mA RCD, but you would not be able to have it cover all of the circuits.
Someone mentioned something about the cables being in 'steel' conduit, not certain in what context, but there's no requirement in BS7671 for anything other than earthed metallic conduit?
In many instances the conduit was used as the CPC, and now due to corrosion the conduit is not good enough to still be used as the CPC.
You may find it very hard to pull out the existing cables and even harder to pull in T&E. It may well be easier to use singles.