Insure one car with Admiral the now.........when your other cars insurance expires you transfer them over to a multicar policy and they charge you pro rata to the end date of your original policy with them.
For example my shed insurance with Admiral expires on the last day in August. The zed has been SORNed over winter so i phoned them to put it back on a couple of days ago and only have to pay £250 to insure it from now until the last day in August when the shed policy ends.
Hope this makes sense