Thats the best solution, breaking out the adiabatic to prove that your none standard 1mm spur to FCU from ring is not really in the spirit of how things are generally done (and would likely prove tricky at EICR time)
But generally for breakers operating on the magnetic release, things are happening so fast that finding I²t yourself doesn't really work, you need to use tabulated figure for the I²t, you could go to manufacturer, or you could just use the worst case figures from BSEN60898:
Theres two tables, one for <=16A and one for >16 <=32A
You'll find just about all current breakers are energy limiting class 3 (look for a three in a square), the table makes reference to the 'rated Ka' if the breaker, and the higher ones have higher I²t, now I suspect thats because if you have a 10ka breaker, the figures provided are those for when its exposed to its maximum rated breaking capacity, one would assume that you could use the 3kA figures for a 10ka breaker installed at a location where its exposed to 10ka but nothing actually says that...
So 3kA. B32 MCB. Energy limiting class three gives you 18000A²S. Which for K=115 gives 1.16mm² which is slightly over your 1mm. but you are not actually going to have a loop impedance of 0.076 ohms at your accessory, il'll be a lot lower, no figures given for lower Ka, but you know is its more than a little bit under 3kA it'll work out fine.
You can run into the earth looking a little bit undersized in 2.5mm twin for the legs closest to the DB if you have a commerial installation with high PEFC with the board close to the origin and C type breakers used.