As others have said, granny charger only off that supply. You’ll need to run a new cable from the meter cabinet or consumer unit.
It would seem most manufacturers want to to also have a 40A breaker these days. While a 4mm and 32A breaker is fine per the regs, if a manufacturer says install a 40A breaker, the regs also state you have to follow the manufacturer direction. So 6mm cable and 40A breaker will likely be required.
You don’t *need* a 100A fuse, 80A is also common and also fine. If you have an 80A fuse, it will not be upgraded as it’s the new standard for a lot of DNOs.
A 60A can be made to work but you’d need to fit a charger with grid monitoring. DNOs tend to come and upgrade the supply as needed after being notified of the charger installation.
Any competent electrician should do a demand test before fitting anyway and advise accordingly.