The conditions that caused reported temperatures down to 2 degrees C will be mysterious to many. So what happened? Crudely, when it's still and cloudless, warm air rises due to convection. This is then replaced by colder air from a higher altitude. That cold air is denser, which is why it falls. In windier conditions it mixes, and when it's cloudy, the air column is capped at the cloud level.
Once the cold air hits the ground, it acts as a fluid, and rolls down the easiest channels. In Brittany there are abundant hedge-banks, and they channel the air towards streams and roads, where the flow is least impeded. The cold air then ponds in lower areas, which contain dense, and cold, fog.
So the coldest air is flowing towards you as you climb, and you will descend into very cold, and wet, air. The result is that it's coldest at 5.30am or so. Those conditions were in place in 2003, 2015 and 2019. Reporting is better now, as GPS's have thermometers.
For the science-minded, its a mixed temperature inversion and katabatic effect. For those who like modelling and maths, here's a paper.
The formation of cold-air pools in valleys and sink-holes during clear nights is a well-known process that has been intensively studied by means of field experiments (e.g., Clements et al. 2003; Steinacker et al. 2007; Price et al. 2011; Lareau et al. 2013) and numerical simulations (e.g., Zängl 2005; Burns and Chemel 2014; Vosper et al. 2014). The strength and depth of the temperature inversion that develops during night time strongly influence the development of the boundary layer during daytime (Schnitzhofer et al. 2009) and therefore also the exchange of heat, moisture, momentum, and pollutants with the free atmosphere above the valley.
https://journals.ametsoc.org/doi/pdf/10.1175/JAMC-D-15-0091.1I live in a bit of a frost hollow, so I'm aware of these processes. Curiously, the UK capital of these conditions is Rickmansworth.
https://www.independent.co.uk/news/uk/home-news/ice-age-returns-to-hertfordshire-1281725.html