That's generally (as in, both in the US [6th amendment] and the UK [Article 6 of the ECHR]) regarding witnesses offering testimony during trials (and the right

That's generally (as in, both in the US [6th amendment] and the UK [Article 6 of the ECHR]) regarding witnesses offering testimony during trials (and the right of the defendant to cross-examine them) - not people giving anonymous tips which might inspire police to do investigations of their own. In the UK, even if a witness is giving direct testimony in a trial there are situations which, for safety reasons, the witness might be allowed to be anonymous: https://www.cps.gov.uk/legal-guidance/witness-protection-and-anonymity https://www.cps.gov.uk/legal-guidance/witness-protection-and... outlines this.