If a police officer feels that he or she has reasonable grounds to assume that a car or van is being driven by a person who is uninsured, or is not licensed to drive the vehicle, then it is likely that the vehicle will be impounded. The usual way that this happens is when the vehicle passes a fixed or mobile camera which reads the number plate details; these are compared to a central database of all the existing UK car insurance policies. If no policy is registered to the vehicle, this fact is transmitted to the nearest police officer. Often this is done as part of a planned operation; the police will pick a road on which it is reasonably easy to stop a car, which is usually one in which there is a 30 mph speed limit. A camera will be set up in a suitable position, with one or more police officers stationed further down the road to apprehend suspects.
The first you will know about it will be when you are flagged down by a uniformed police officer, who will probably ask you to produce evidence that you are licensed and insured, and if you cannot produce this evidence the vehicle is likely to be seized. It will eventually be loaded onto a flat back truck and taken away to a police pound; you should then be provided with a form numbered 3708 which spells out the steps you have to take in order to get your vehicle back again.
Unless you have good reason for requiring transport, such as being elderly, having a medical condition or carrying young children as passengers, it will be your own responsibility to find your own way back home. If there are any valuables in the car you would be well advised to take them with you at this stage; if your vehicle is eventually scrapped or sold by the police it is likely that these will meet the same fate. If you cannot remove these immediately, you should arrange to visit the police pound as soon as possible to do so, even if you are not able to remove the vehicle itself.