In Eskadalemuir, Dumfries and Galloway, the mercury dropped as low as 5.9C.
In England, racing was abandoned at Lingfield Park in Surrey after an overnight temperature of minus 5.5C (22F) made the track frosty, while Benson in Oxfordshire saw a record low this winter of minus 8.1C (17F).
Drivers have been warned of icy conditions.
Met Office forecaster Mark Wilson said: “We would warn people to be aware really and just be considerate of the fact that there could be some icy patches which are on untreated surfaces like pavements, cycling paths and minor roads.”
Conditions are set to get milder into the weekend, with daytime highs of up to 10C (50F) in southern parts of the UK, he added.
Scotland and the far northern isles will be slower to catch up, lingering around 5C (41F) before lifting to 10C (50F) by Monday.