The Met Office released information this afternoon that a weather station recorded 18.7C in Achfary, on the North Coast of Scotland on the 28th of December 2019. It will need to be quality checked, and if verified it will become the highest temperature recorded during the month of December in the UK.
The following video from the Met Office gives more information.
You might think this part of the UK is the least likely place to see the highest temperature recorded during December. However, if winds are strong enough and flow over high ground, i.e. the Scottish Highlands, on their descent they can loose humidity and the associated air can significantly increase in temperature. This Meteorological phenomenon is known as the Foehn effect, and is common in the UK during the Autumn and Winter months.