Background: Adherence to UK dietary guidelines is poor, yet no evidence exists regarding population-level knowledge of these guidelines, or the barriers and facilitators to adherence. This study aimed to characterise knowledge of UK dietary guidelines and perceived barriers and facilitators to adherence in a nationally representative UK sample. Methods: A cross-sectional survey was administered to 1003 adults recruited via Prolific, matched to the UK population by age, sex, and ethnicity. A 22-item knowledge questionnaire assessed awareness of the Eatwell Guide and broader Dietary Reference Values (DRVs), with both strict and liberal scoring applied. Perceived barriers and facilitators to adherence were assessed using custom questionnaire items informed by the COM-B model and TDF framework. Results: Knowledge of Eatwell Guide recommendations was moderate under strict scoring (53.3%) and improved under liberal scoring (72.5%), despite nearly half of participants reporting no familiarity with the Eatwell Guide. Knowledge of broader DRVs was poor using strict scoring (17.9%) but moderate with liberal scoring (58.9%). The most commonly reported barriers were social (e.g. celebrations), environmental (e.g. access to unhealthy foods), and psychological (e.g., mood). The most strongly endorsed facilitators were economic (e.g. cheaper healthy foods) and health-related (e.g. motivated by weight and mental health). Conclusions: These findings suggest that whilst knowledge of UK dietary guidelines is reasonable, individualised behaviour change approaches alone are unlikely to be sufficient. Meaningful population-level improvements will require complementary structural changes to the food environment.