The car would be responsible since it was fully autonomous. That's the whole idea of autonomous driving: the car can drive without anyone in the vehicle. So, the autonomous car is the driver. Hence, it is responsible.
The liability is for autonomy, not for driver assistance. Companies will perform the same calculation as they always do: if income > cost then do it. Being in an AV is just like riding in a taxi. If the taxi driver crashes, you aren't at fault. If the AV crashes, you aren't at fault. For L3 or L4 the AV has to figure out whether autonomous operation is safe and be able to give enough warning to the driver or monitor to switch back to assistance. And yes, FSD will be deployed as driver assistance. That's why it's FSD, not FA.
Now that UK is not part of EU, if they pass some laws that other countries/blocks don’t pass - manufacturers will just not deploy the features. Ofcourse in this case law is just codifying something fairly obvious.