What can be 45 minutes in a Zipcar can sometimes be more than 2 hours by public transport, and sometimes impossible given some time constraints and rail engineering works.
Local car rental companies do rent cars by the day, and can sometimes be cheaper, but they come with more complications: admin faff, parking problems, and inflexible 8am to 8am rentals mean that a simple 3 hour Zipcar rental from 6am to 9am becomes a 2 day car rental, or needing a car on a Sunday means a 2 day rental from Saturday to Monday as none of the local car rental companies seem to open on a Sunday. Some of the major ones (Hertz/Avis/Budget/Enterprise/Thrifty) do but then I'm travelling an hour to pick up the car.
If Zipcar does shut down in the UK, and no-one jumps in to fill the void, then I may have to buy (more likely lease) a cheap car and add to the clutter on my local street with something that is only used 4-5 times a month. More convenient for me, costs about the same year on year, but terrible use of resources.