Solved

How to get lvl 20 driver

Discussions between mods and iGP Team members suggests that driver attributes are the important factor not the level. So a driver who has 20 across the board at L18 will not be disadvantaged against a L20 with the same attribute scores.

In fact the lower level driver does have the advantage of faster training because any driver whose level is the same as the Team/Manager has a training effectiveness of 3% per session whereas lower level drivers gain more per button press.

But if you really want your driver to achieve L20 I was recently given a workaround from a very experienced manager. I haven't tried this but the manager in question is an extremely reliable source of information so I have no reason to doubt it. Apparently you should train your driver to 20 in everything except one of the "Mental" attributes which you should leave at 19. Wait until the driver's experience rating goes to 20 then train the final attribute from 19 to 20 and apparently the driver level also jumps to 20.... But it doesn't make them any faster LOL.

Finally, the iGP Team are aware of this issue and it's on their list of things to resolve.

Of course it checks out. I first tested out the theory myself before telling the other mods about it ;)