New cable won’t reach the CU unfortunately. I’m going to take it to a FCU and run it from there.Is the new cable able to reach the CU? If so a separate MCB to feed the garage CU with (I presume) RCD or RCBO protections is better.
If it won't reach, but you could run SWA from house CU to where it currently ends up then you could join them using a resin joint. Bulky and a bit weird for indoors use but it is maintenance free!