I was told by an electrician that I could spilt the tails after the 100a isolation switch via a Henley block . From the Henley block I would use 25mm2 tails to a Garage Unit with a RCD of 40 a and 2 breakers or 32a and 6A.You need overcurrent and fault protection at the start of the circuit which feeds the garage.
What are you installing after the 100A isolator to protect the circuit which feeds the garage?
What type of cable are you using and how long is it?
I suggested using an armoured from the current consumer unit ( via a 40a breaker) into the back of the new Garage unit. But I was told better to spilt the tails.