Not every vehicle, nor indeed every driver, can be accepted for short-term or one day car insurance but the majority of applications are successful. The conditions below are the generally accepted ones but they do vary from one insurer to the next so they should be viewed as approximate guidelines, not firm stipulations.
Firstly, let us look at the car you wish to insure. Every short term insurer has a fixed list of vehicles which can be accepted and, although the majority of modern UK cars are acceptable, if yours is not on this list you cannot simply find the nearest equivalent car, the fact is that unfortunately that particular insurer will not cover it. The vehicle would have to be roadworthy, kept and originally registered in the United Kingdom, and have a value of at least £1000 or so and a maximum of £40,000. No more than seven seats should be fitted and the vehicle should not used to carry passengers for hire or reward (although cost sharing is permitted). It should either belong to you or be borrowed (not hired) and driven with the owner's express permission.
Secondly, the driver should be aged between 18 and 75 and must have held a driving licence for at least six months in the case of a UK driving licence, or one year in the case of an EU one. A clean licence is obviously best but there should not have been more than six penalty points imposed over the last three years, no disqualifications within five years and no more than a single fault claim within the last three years.
If in doubt, get a quote! If your circumstances are not within the insurers' guidelines you'll be informed of this but if you do get a quote it will mean that your details are acceptable. It should only take you a minute or so to get a quotation so why not go for it.