In uk.d-i-y, Mark wrote:
Hello,
I want to install a secondary consumer unit so that I can do some
outdoor wiring. I'm happy with the technicalities of how to wire from
the consumer unit.

I couldn't tell from your description whether the secondary CU is going
to be nice and close to the existing CU and the main service fuse. If
it is, the Henley block is the way to go, and the 100A isolating switch
is a Good Thing too. But if the secondary CU is some distance away (more
than a couple of metres), it's both too expensive and not safe to run
gert long 25mmsq tails to it relying only on the service fuse to protect
them. Rather, you'll want to take a sensibly-rated way out of your main
CU (30A, 40A, in the extreme 63A) and feed your physically-remote CU for
the outdoor circuits via a suitable "submain" cable - depending on
max loading, distance, need for mechanical protection and so on, that
could be beefy twin-and-earth (6 or 10mmsq), SWA, or the useful halfway
house of Supertuff.

"Normal" circuit-design calculations will tell you what cable and
overcurrent protection to use.

HTH, Stefek