Release - charity offering legal advice for those charged with drug-related offences, those who are concerned about their or someone else's drug use, or general questions about the effects of drugs and harm reduction.
TalkToFrank - information about drugs endorsed by the UK government. Also offers advice regarding the use of drugs via their helpline.
ReagentTestsUK - sells various testing kits. Cryptocurrency accepted and anonymous delivery options offered. Carefully note the limitations of reagent tests detailed on the site, especially that reagent tests usually do not test for purity, and indicate presence rather than absence.
Wedinos - Wales based charity offering a drug-testing service (far more comprehensive than that offered by reagent tests). Test results are public, but do not infer about the contents of your drugs based on their pill or blotter design, pills/blotters with the same design can have radically different contents. Sometimes pill designs deliberately mimic that of well-known suppliers, and plain blotters of a particular design are easy for suppliers to obtain.
PsychonautWiki - community-based resource concerning recreational drugs. Contains safety and dosage recommendations, as well as a list of common subjective effects. Note that this is a wiki and is not necessarily reviewed by experts - though citations should be given. Occasionally the effects, risks and common dosages of certain novel or uncommon drugs are suggested from that of similar drugs rather than from specific experience with the drug being discussed. Nonetheless, descriptions of certain subjective effects are hard to find elsewhere.
Drug Science UK - charity run by David Nutt, hosts research on psychedelic drugs and offers professional training on drugs.
Erowid - documents subjective experiences of and contains harm reduction information pertaining to recreational drugs. Contains some safety and dosage advice.
Reddit can be useful for reading about subjective experience and harm reduction, though caution should be made on any speculation of the safety of particular novel drugs or combinations thereof that may be found (even those which seem informed).