Probably nitrous oxide and xenon wouldn't come up on a hair or urine test. Ethanol, caffeine and nicotine are presumably allowed if your location (london) is accurate. Nutmeg is also legal but it might cause some misleading test results.
[Edit: were you wondering if DMT would show up on the tests? Because if you were thinking it might be something you could use like a drug - well,
try reading a few more threads on the forum.]
You could just not take any drugs - that would be simplest. Or you could tell the college that their drug testing policy is an infringement of civil liberties in general and that you won't be attending. Which is more important to you - 'getting high' or getting the education that this college offers?
Is the college placed linked to some other conditions such as unemployment benefit or probation conditions?
Hopefully you've read this already:
https://wiki.dmt-nexus.me/Attitude_Page “There is a way of manipulating matter and energy so as to produce what modern scientists call 'a field of force'. The field acts on the observer and puts him in a privileged position vis-à-vis the universe. From this position he has access to the realities which are ordinarily hidden from us by time and space, matter and energy. This is what we call the Great Work."
― Jacques Bergier, quoting Fulcanelli