The correct MCB to use is the one you selected after calculating the maximum demand after diversity and before calculating the size of the cable you used.
What method did you use to decide that 6mm cable was correct for your installation?
I know tests will determine what size to use from house and what cable to use.. It's the 63 amp main switch, the 6mm cable I chose was simply because I planned on using 63amp mcb ,
For what it's worth I'm gonna go 32a mcb with 4mm cable to shed saves all the sarcasm and I AM choosing 4mm because the circuits will never be in full demand... 2 spots and 2 double sockets