There area lot of radically altered cars out there these days like focos etc that are still running on the original donors ID rather than going through an IVA inspection.

The DVLA will be the least of the owners problems if such a car is ever involved in a fatal crash, I imagine the CPS would come down on the owner like a ton of bricks with some serious charges.

Its just not worth the risk in my opinion not to get them checked out and certified.