You don't usually fail because of one major fault, the examiner will have taken a view over the whole time on the test and made a judgement. He then list some of the things that went wrong. At the end of the day you need to inspire confidence in the instructor, it's not always black and white.
Andi.
As others have said - you absolutely do fail for one major fault.
https://www.gov.uk/practical-driving-test-for-cars/your-driving-test-result
Sometimes I think it's funny how people think that showing a high degree of competence means the driving instructor "will give you a better score" when likely all you're seeing is a "better score because you did less wrong BECAUSE you have a high level of competence".
Still, we're all human. Being smooth and confidence will no doubt be picked up on by the tester and it wouldn't surprise me if this was marked just like every other aspect.
On topic - 1 foot behind any vehicle is too close. Hell, even parking with only 12 inches of space is asking for a bump if the person next to you doesn't notice.