In general, the UK's weather is mild throughout most of the year. This is because of the sea which surrounds the British Isles (Britain and Ireland). The sea warms up and cools down more slowly than the land, keeping winters relatively warm but also making the summers cooler. The UK also benefits from the warmer sea of the Gulf of Mexico (the Gulf Stream) that keeps the sea off the west coast slightly warmer than that of the east.
Although the UK doesn't experience very hot summers, when it does get hot from time to time, it feels rather uncomfortable due to the lack of air conditioning, so it's advised to bring light cool clothing.
During the winter season, everywhere will be well heated but you will still need an adequate layer of clothing to keep warm outside. During the summer it can still get quite cool in the evenings, so a light jacket may be necessary. The images below help to show the average temperatures in the summer and winter.
Copyright © 2017 UKEAS. All rights reserved.