I think the car forms part of the protection. You don't get out at all.
I haven't had a test in the UK but I've done both forms of drive through test here; where you get out of the car to see the tester and where they reach through the window. The former has never been mentioned as a risk anywhere I've seen.
@atz started
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I think the car forms part of the protection. You don't get out at all.