This is from my local council but it will apply elsewhere as well:
https://www.midsuffolk.gov.uk/assets/Environment/abandonedvehicles.pdf
You can find relevant information from your own local authority by entering your postcode here:
https://www.gov.uk/report-abandoned-vehicle
However just because a car hasn't moved for months, it's not necessarily abandoned. The owner might, for example, be in a care home, working abroad or serving a prison sentence.
There's a car under a tarpaulin in the communal parking area just a few yards from my house. It's not abandoned because the tarpaulin will be replaced by someone if it gets tatty or blown out of position by the wind. However the car hasn't moved since I came to live here 28 years ago (when the state of the tarpaulin that covered it at that time suggested that it had already been there for a decade or so).