You can register your car with TfL and you'll be automatically charged if you pass a ULEZ camera. Unless there's a camera in your street (unlikely) you won't be charged for small movements or parking.
There are no rules about not keeping a non-ULEZ car parked in the zone. Your local council might charge you extra for a residents parking permit (or other paid parking) if it's one of the ones that's militant about pollution, but they can't do anything else.
The number of 90s cars still parked outside houses here (inside the 2021 ULEZ) suggests keeping an occasional use non-ULEZ car is perfectly viable.
You can register your car with TfL and you'll be automatically charged if you pass a ULEZ camera. Unless there's a camera in your street (unlikely) you won't be charged for small movements or parking.
There are no rules about not keeping a non-ULEZ car parked in the zone. Your local council might charge you extra for a residents parking permit (or other paid parking) if it's one of the ones that's militant about pollution, but they can't do anything else.
The number of 90s cars still parked outside houses here (inside the 2021 ULEZ) suggests keeping an occasional use non-ULEZ car is perfectly viable.