Is it an offence to obstruct someone's drive?

The law says you are entitled to access to the highway. It does not say you are entitled access to your driveway. Therefore if someone parks in a dangerous position denying you access from the driveway to the road, they commit the offence of dangerous parking. You'll need to convince your local plod that there is an element of danger created by this person. Not easy to prove in my experience. Usually it is only after a collision has occurred that a person who parked the vehicle in a dangerous position was prosecuted and convicted.