Failing to name the driver

I received a letter for an intention to prosecute from Warwickshire Camera Enforcement Unit for driving 59mph in a 50mph zone. I asked for photographic evidence to help me identify the driver. I promptly received a letter stating that there wasn’t such evidence and I needed to supply the name and address of the driver. I wrote to them and stated that I believed it was my step brother who had been here on holiday. Again a swift response advised me that because I had not filled in their form and had written them a letter they would pass a case onto the courts. I called them and they gave me 2 weeks to fill in their form confirming who the driver was. I filled in their form with the name and address of my step brother. They again promptly wrote back stating that they checked and no insurance cover was on my vehicle for my brother on the day of the offence and said if I do not it will result in court proceedings. My argument is the car was taken without my knowledge. Also they are asking for evidence that my brother was in the country such as flight tickets or a stamped passport? I have 10 days to respond from today.