The cold weather has been caused by weather sweeping in from Russia. Winds from Siberia have pushed in from the east, causing the the temperatures to drop to minus five, with a wind chill as low as minus 12. This has been caused by the northern polar jet stream which has twisted its direction unexpectedly, drawing in cold air as it crosses the country. The last time happened was 2013, but the UK has not experienced such low temperatures since 1991.
The south-east has gotten off fairly lightly with the north and south-west affected the most.