The lowest temperatures of 2014 for England, Wales and Northern Ireland were recorded overnight.
Katesbridge in County Down hit -8.8C (16.2F) while Benson, Oxon, fell to -7.6C and -6.7C at Pembrey Sands, Dyfed.
Latest figures show the UK's coldest temperature of the year so far was -9C in Cromdale, Moray, on Saturday.
Meanwhile a Met Office cold weather alert for northern England and the Midlands has warned of health risks for vulnerable people.
BBC Weather said Scotland saw temperatures fall to -7.7C in Braemar, Aberdeenshire, overnight from Sunday to Monday.