Is it allowed to extend an old circuit using black and red wiring with new blue and brown. (the grey cable, not the rubbery black stuff)

Isn't the CPC wire size bigger in blue and brown?

I have several circuits (sockets and lights) running from a 'submain' fusebox,
I'd like to use them but powering each from a separate MCB and blue and brown cable coming from the consumer units at the other end of the house.

Thus all fuses and trips will be in one location.

But is it allowed to mix different cables in one circuit?
And how would I calculate the maximum size of MCB per circuit?

And for lighting circuits can I run the main power line in 1.5mm and then spurs to individual lights in 1mm squared?

Eventually I want to do full paperwork and testing and try to get it all approved by the council under building regs.

George