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You're almost right re the legality of ayahuasca, however when you boil in water you in effect make a water extraction of dmt. Granted, it is a crude extraction but an extraction nevertheless.

Imagine this; police raids and you're half way boiling your soup. You basically make an acid extract that (depending on charges a prosecutor may want to press) can be used in an A/B to make a pure extract. In addition, there're some grey areas about preparing something for human consumption, so preparing aya to drink (as opposed to preparing aya as a dyestuff/fertiliser/insecticide/whatever) may call for charges.

In any way, these are almost never issues in UK where most prosecutors are not really bothered too much about dmt and where a good lawyer can easily sort things out. They are more interested in weed, diamor and coke. There had been a case where dmt, as well as an extraction-in-progress was described as "novelty drug" and charges were not pressed.