The guy on Radio4 at lunchtime was saying that 6 cars would be tested in Greenwich later this year. That makes it sound like they already have the cars in mind to test.
But why do the first tests on small city streets? Surely you start out on something big and predictable like the M1. Set it going at 56mph and it's a (relatively) simple task. Certainly compared to parked cars, kids, cyclists, pigeons, and that bloody great line they set the clocks by.