Cascading Consumer Units.

Would be a better option in most cases to split the supply via a Henley block to feed both CUs directly, or replace the existing CU with a larger one if space permits.

Just my off-the-cuff opinion!

Yes, but you are obviously limited by the overall rating of the original CU and of the fuseway you use to feed the sub-CU.

What sort of circuits are you wanting to add? It wouldn't be a good way to turn a lighting circuit into three electric showers, for example.

Owain

This is better, but the OP's suggestion is also allowed. Would suggest using a BS1361 cartridge fuse on the way which feeds the second CU, as it will offer better discrimination than daisy-chaining MCB's, where you'll likely find they both trip on short circuit faults.