Decreasing floral resources as a result of habitat loss and fragmentation is one of the key factors in the decline of pollinating insects worldwide, with the resulting impact on food supply and biodiversity receiving increasing global concern. A detailed understanding of which plants honeybees choose to use throughout the season is lacking in the literature and is vital to elucidating honeybee foraging behaviour, and to provide effective recommendations for suitable forage to support hives. Here, using DNA metabarcoding, honeybee foraging has been characterised throughout the season for hives set within a diverse landscape, revealing the plants that are most important to honeybees when offered a wide variety of species. To set this detailed foraging study within a wider UK context, honeybee foraging is then assessed for hives across the UK for the first time since 1952 and in doing so we evidence national scale changes in nectar provision over time. The results of this project provide scientific evidence to support beekeepers, as well as informing landscape level decisions in providing and improving habitat for pollinators. To support this work from a strong knowledge base and provide a high-level of confidence in the taxonomic identification of this and future DNA metabarcoding data, a reference library is created for the flowering plants and conifers of the UK, for three DNA barcode loci. Species coverage and discrimination is assessed for both the native plants alone and when naturalised and horticultural species are included. This work provides a high-quality resource for the honey DNA metabarcoding research presented here, as well as the many current and future applications of plant identification using DNA barcoding.