if I were to install a new consumer unit in a garage which has a 100amp main switch supplied by means of 6mm armoured cable, could this cable be connected via a splitter to the incoming supply of the house,
Depends how long it is, how it is installed etc.
It is a bit backwards, normally you'd start from what you are going to connect and work out the cable size.
You'll need a few of them as you also need to familiarise yourself with the regulations.
The reason you must to do it all properly, apart from safety, is that it's notifiable, so if you are going to be DIYing instead of using an electrician there's a lot that you have to get right.
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