try calling a classic specialist like footman james and talk to them nicely.
A classic insurance specialist understands that you need a collection of cars insured (all classics ) and that you can only drive one at a time. You should be offered a policy that covers all the cars at a single premium and each added car costs a fee to cover the extra paperwork. Each car has its own certificate and is treated as a single car. You will not receive any no claims discount for this service and they will not take any into account.
If you want to cover modern cars as well, then a fleet poliy would be more suitable, but a fair bit more expensive.
I run a classic policy for the mini's, and a modern single car policy on a Vauxhall Astra, this allows me to dive any of my own cars, my wife can drive the Astra as a Spouse and i can drive anyone elses car on third party only cover. I have found this works perfectly for us so far. But i am now considering taking a fleet policy for the business and registering all the vehicles and all the drivers in my family, so my Son will also benefit from this service as he wishes to have two cars available for him to drive.