Background: As a policy option to reduce consumption of alcohol and the harm it does, on May 1, 2018, Scotland introduced a minimum price of 50 British pence (p) per unit of alcohol (8 g) sold; Wales followed suit on March 2, 2020, with the same minimum unit price (MUP). We analysed household purchase data based on bar codes to assess the impact of these policy options in the medium term for Scotland and in the immediate term for Wales. Methods: For these location-controlled, interrupted time series regression analyses, the data source was Kantar WorldPanel's household shopping panel, which, at the time of our analysis, included 35 242 British households providing detailed information on 1·24 million separate alcohol purchases in 2015–18 and the first half of 2020. With no data exclusions, we analysed the impact of introducing MUP in Scotland, using purchases in northern England as control, and in Wales, using western England as control. The studied changes associated with MUP were price paid per gram of alcohol purchased, grams of alcohol purchased, and amount of money spent on alcohol. Findings: In Scotland, price increases and purchase decreases following the introduction of MUP in 2018 were maintained during the first half of 2020. The difference between Scotland and northern England in 2020 was a price increase of 0·741 p per gram (95% CI 0·724–0·759), a 7·6% increase, and a purchase decrease of 7·063 g per adult per household per day that an alcohol purchase was made (6·656–7·470), a 7·7% decrease. In Wales, the introduction of MUP led to similar results. The difference between Wales and western England was a price increase 0·841 of 0·841 p per gram (0·732–0·951), an 8·2% increase, and a purchase decrease of 7·052 g per adult per household per day that an alcohol purchase was made (6·463–7·640), an 8·6% decrease. For both Scotland and Wales, reductions in overall purchases of alcohol were largely restricted to households that bought the most alcohol. The introduction of MUP was not associated with an increased expenditure on alcohol by households that generally bought small amounts of alcohol and, in particular, those with low incomes. The changes were not affected by the introduction of COVID-19 confinement in the UK on March 26, 2020. Interpretation: The evidence base supporting the positive, targeted impact of MUP is strengthened by the comparable results for Scotland and Wales. The short-term impact of MUP in Scotland during 2018 is maintained during the first half of 2020. MUP is an effective alcohol policy option to reduce off-trade purchases of alcohol and should be widely considered. Funding: None.