The large'ish 1930s house I moved to a few weeks ago has some
"interesting" wiring that I need to sort out over the months ahead.
The incomer is in the (attached) garage.
There's a Henley block that splits the tails into 4. One of these goes
to a 16 way plastic CU in the garage, two go to 4 way DBs (to sheds,
elec shower and elsewhere) in the garage, and 1 goes (via an isolator)
to a sub-main that goes to an 8 way (wire-fused) CU in the house for the
sockets. One of the MCBs in the 14 way board feeds a second 8 way
(wire-fused) CU in the house for the lights.
The plan is to rationalise this mess by replacing all the garage CUs
with a single 21 way MK sentry metal CU and to replace the two ancient 8
way wylex CUs with a single 16 way metal CU (and a single sub-main). My
questions are about the sub-main and shed wiring.

The sheds each have small CUs for lighting and a few sockets, each shed
has an earth rod. My guess is the load in each shed will never exceed
20A so I plan to run a cable to each shed from a non-RCD'd MCB in the 21
way CU. The local shed CUs will have RCDs. Any problems with that?

I plan to feed the sub-main to the 16 way CU from an isolator connected
to the meter tails. Any problems with that?

I need to investigate a bit more but I think the sub-main cable is
currently 16mm2 T&E with a 4mm2 grn/yel wired in parallel. How do I
calculate the necessary size of CPC for the sub-main?

As an aside, I think I've only got one run of rubber insulated cable to
replace but I was surprised to find some disconnected lead covered cable
in a shed, together with some original 1930s switches and sockets.