Might this help?
Lowest minimum temperature: Lowest value of the daily minimum temperature for each calendar year, averaged over climatological reference periods.
UK: 1961-1990 average -8.5 °C; 2008-2017 average -6.8 °C.
Shown us that the lowest daily minimums on average have been rising, even with very cold spells in perfect synoptic conditions? Might this tie in with our ability to breach record low temperatures?