It does appear to be true that a car with a green flashing light on top indicates a doctor on an emergency call. drivingtesttips.biz (for example) says, somewhat curtly: "Green Light: Doctor on emergency call".
But it doesn't say so in the Highway Code.
All the Highway Code says (in Rule 219, Emergency and Incident Support Vehicles) is, "[y]ou should look and listen for ambulances, fire engines, police, doctors or other emergency vehicles using flashing blue, red or green lights and sirens or flashing headlights ... "
In the specific context of doctors using green flashing lights, Wikipedia refers to a page on the UK Government website, legislation.gov.uk. But this is a Contents page and I have failed to find the page where it says doctors can use green flashing lights. (This doesn't mean I don't believe it's in there somewhere!)
I can only conclude that the question setter thought that if they included the words "According to the Highway Code" in the question, it would make it look as if they knew what they were doing. If anything, it's actually had the opposite effect.
© Macclesfield Quiz League 2018