Even though UK weather is unpredictable, it is seldom extreme. The average temperature ranges from 9–18 degrees Celsius in summer and 2 and 7 degrees Celsius in winter.
But temperatures can reach around 30 degrees Celsius in case of a heatwave and often drop to just below 0 degrees Celsius in winter. Most houses, buildings, trains and buses have good heating systems as such no need to worry.
There is little difference between the regions of the UK, but you will experience more snow, rain and wind in northern and mountain areas.