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If you are caught taking drugs then you are by definition caught in possession of a controlled substance.
If you are arrested for being high when not in possession the police would have to prove that you were, at some point, in possession of the substance that got you high. I'm sure they've got bigger fish to fry. If they try to prosecute you for the act of being high then it's pretty easy to say something along the lines of; my drink was spiked / someone gave me a cookie to try it tasted a bit funny.
IANAL
Well IANAL, but gov.uk seems to think it is -
Drugs penalties
You can get a fine or prison sentence if you:
take drugs
carry drugs
make drugs
sell, deal or share drugs (also called ‘supplying’ them)