Christian McArdle wrote:

6kA is normally enough.

Agreed. Although the fault level (PSSC) on a house service can be as
high as 16 kA, it's rarely anywhere near that.

Most MCBs are 6kA anyway, with some 10kA.

Yes, but the complete MCB CU system will have been type tested to BS EN
60439-3 at 16 kA and given a conditional rating - conditional, that is,
on the presence of the supplier's 100 A (BS 1361 Type 2) fuse to provide
backup protection. IOW even when the MCBs' breaking capacities are
exceeded you have an assurance that the supplier's fuse will clear the
fault without the CU blowing itself apart (at 16 kA the magnetic forces
are huge) or catching fire. You can't necessarily assume that this will
be the case with an old Wylex board fitted with the wrong type of fuse.

A quick check with a loop tester should indicate compatibility, if
you happen to have one lying about...

Of course I do :-). The lowest impedance domestic mains supply I've
ever encountered was about 0.05 ohm (phase-neutral loop) so c. 5 kA
fault level.

--
Andy