To take out traders car insurance you will need proof of trading and this must be kept for the duration of your policy.
You will need the license number and the a policy where you can add any unnamed driver, which will be more expensive and still comes with restrictions such as age and convictions.
During your policy you should also inform your insurer of any other permanent vehicles and trade plates owned by you that need covering. These will then be added to the Motor Insurance Database. You should also add any temporary vehicles such as courtesy cars, short term hire vehicles or vehicles you need on a regular basis as this could save you a lot of time. Adding these vehicles, regardless of the period for which you use them can minimise your chances of being stopped by the police as they have a record of valid insurance on the MID.
Once you have taken out a policy you will need to add all the cars that come under your care to the MID temporarily and remove them when they are no longer in your possession. This may require receipts of activity of when vehicles are bought or sold so the MID knows what you are trading at all times. However you will not need the vehicle details upfront of course, just roughly how many you will be handling and their value to take out motor traders insurance.