Oh, I forgot the second point... My property has ample on site, off street parking. Two blocks away, another local host lives above a shop in our shopping district and so parking is limited and guests must park one block away, but up a rather steep hill... I KNOW for a fact that guests have booked with me over her. They told me so.
For you, how many unbooked nights are you hoping to fill? If you are mostly booked, I wouldn't worry about a paid parking spot. But if you are struggling with occupancy, and you can bump the rates by £ or two to cover the cost, it may be what you need to encourage booking. If I saw reserved spot, even off site, I would totally keep a listing in my choices. In fact, that was how two of my three hosts handled it on our recent trip. One of them, the spot was right outside the house, one was a bit away, but as I parked the car the entirety of my stay, I didn't care. A reserved spot is key, knowing that there will be a place no matter what time I come and go, eliminates a lot of stress.