Pre-registration stems from the ability to tap into significant buying power of companies that purchase hundreds of cars a year: a large fleet company, for instance, will receive discounts of perhaps as much as 30% off the price of a Ford Mondeo because Ford want to sell as many cars as possible to them. The private buyer can benefit from big discounts by purchasing a car that is first registered in the name of the fleet company to receive that 30% saving - although it naturally means that they will be the second registered keeper of the vehicle.
AutoeBid has a help page which outlines the benefits and downsides of this, which can be found by clicking here. What Car? has also produced a useful guide to explain pre-registration, concluding: "As our research has shown, this is a great way of picking up new car with a massive discount and in some instances you might not even wait for it to be delivered because it's already on the garage forecourt."
AutoeBid always offers a first-reg price as well as a pre-reg deal where applicable on new cars, so you can compare one with the other; to check a price on your chosen car, simply click the following link.