Use a consumer unit approved for use in the UK, and don't mix and match components inside from different manufacturers.  You're relying on the manufacturer of the CU to declare it's safe for a very brief 16kA overload.

Use a consumer unit approved for use in the UK, and don't mix and match components inside from different manufacturers.  You're relying on the manufacturer of the CU to declare it's safe for a very brief 16kA overload.

Not really, it is better to consider that you are relying on the 100A company fuse to open first and thus limit the down stream energy reaching  the breakers and protect them for the very small fraction of real faults (if any, ever) that draw the full PSSC. Almost no real fault is zero ohms (if it was there would be no heat light or sound as no energy would be dissipated), and also within a meter or so of the CU so not limited by the resistance of some outbound cable....

The risk arises not so much from mixing MCBs, but rather more if you were to bypass the company fuse with bits of coat hanger wire - which I hope you don't.

Mike.