In your case, if the cable connecting the house consumer unit to the garage consumer unit is rated at 40 amps or more (and has a 40amp breaker in the house consumer unit), then you could install the charger in the garage by adding a 40amp breaker into the garage consumer unit and wiring the charger, using 40amp cable, to the 40 amp breaker. The 100amp fuse at the input stage is adequate for car-charging using single -phase 7kWh chargers.
My situation is a little different:
At the mo, I have an Ohme (portable) smart charger with a 32-amp Commando plug on the other end - https://store.ohme-ev.com/shop/ohme0402gb001-ohme-go-commando-to-type-2-uk-5?purchase=cable#attr= I didn’t pay the current price for it…
This allows it to be used as a 32amp/7kWh charger like any domestic single-phase charger. The plan is to get the 60amp house fuse upgraded to 100amp, have a 40amp breaker fitted and 40amp cabling from the house consumer unit to a switched (interlocking) 32amp commando socket on the exterior house wall.
I have a 13-amp 3-pin plug to 16amp commando socket converter cable AND a 16amp commando plug to 32amp commando socket converter cable, which both connect together and then into the Ohme’s 32amp Commando plug. The Ohme is set, by Ohme at my request, to only pull 10amps. So I’m basically using the Ohme and all the converter sections to connect to a domestic 13amp exterior plug temporarily when charging at home. I then unhook the whole thing and throw it into the Ohme bag in the car. This selection allows me to charge anywhere at 10 amps, with either a domestic 3-pin socket (someone’s house), a 16-pin Commando socket (campsites) or a 32amp Commando socket (industrial). Although slowly - it’s basically acting as an elaborate granny-charger at 2.3kWh
It won’t be until I get the 32amp Commando hardware installed at home, when I’ll then up the Ohme’s charging amps to 32 to make it a 7kWh charger. So quicker charging.
But I’m in no rush to jump. Work has just upgraded from 4 7kWh chargers and installed 8x 3-phase 22kWh chargers which can be used for free. So the Ohme set-up really is for emergencies only now.