Can you use it to tax a car?
It's theoretically possible but very unlikely. However despite the title one-day car insurance is not limited to a single day, you can buy anything up to 28 days cover and realistically you would probably need to allow about a week. Why? Well, although officially you should be able to take a downloaded short term car insurance certificate to your local GPO and buy a tax disc over-the-counter but despite these documents being officially acceptable this is still left to the discretion of counter staff and some treat them with suspicion. This is not a terminal problem however since you can tax a car online or by telephone and when you do this the automated system checks on whether or not the vehicle is currently insured and MOT'd. When you take out a short-term policy online the database that this automatic system relies on may take two or three days to update so you could find your application turned down initially, although it should be accepted a day or two (or maybe three) later.
So, to summarise; a one-day policy may be enough, but if taxing the car is an important factor for you it would perhaps be better to buy a slightly longer policy. You can get a quote for this by clicking the 'quote' button.