High-resolution river flow data is essential for modelling flood dynamics and assessing hydrological change. In the UK, there are a wealth of sub-daily flow records that have been collected for more than 70 years. However, they remain scattered across multiple agencies and lack consistent quality assurance, limiting their use for large-sample and national-scale analysis. This paper presents UK-Flow15, a quality-controlled, 15-min, national scale, flow dataset for the UK, based on records from over 1,300 gauging stations and more than 1.8 billion observations. Data were collected via APIs and in conjunction with UK measuring authorities, then compiled into a single national dataset. Duplicate timestamps and inconsistencies in temporal resolution were systematically identified and resolved using a combination of automated filters and manual review. A comprehensive quality-control framework, specifically tailored for the dataset, was then applied to identify and document data anomalies; the full methodological development and evaluation of this framework are presented in the companion paper of this series. The final dataset is accompanied by transparent documentation of flagged issues and a suite of metadata files detailing data resolution, QC outcomes, and all processing decisions to support traceability and user interpretation. We demonstrate the dataset’s utility through a practical case study by systematic removal of unreliable data, interpolation of anomalous spikes, and manual verification of significant high-flow and truncation events. These steps illustrate how the dataset can be effectively curated for detailed hydrological research and operational applications. This publicly accessible, robust, and transparent dataset significantly enhances capabilities for sub-daily hydrological research in the UK, offering essential resources for improved flood prediction, management strategies, and policymaking.