Can't see how it's illegal. I did this with motorbikes and was running around on my dad's ZZR 1100 when I was 21.

As long as the car exists, and has tax and MoT, then no problem. Checked the small print on my current policy that allows this. The important thing is that you say that you are only a very occasional driver if you have a crash. If you say you are the main driver - you are scuppered.