British Isles Plant Hardiness Map
The United States Department of Agriculture invented plant hardiness zone maps and plant classifications so growers could see if a plant would survive temperatures in their region of America, a country with various types of climate, from temperate to desert.
The heat zones were later applied to other maps, including Britain & Ireland. As you will see from below, the mildest zones are in the west, and on the coastlines, with the coolest areas being hilly areas, such as the Peninnes.
Overall the British Isles are much milder than they should be thanks to the Gulf Jet Stream that blows warmer air in from the Atlantic Ocean. For example, Glasgow and Moscow are both 55 degrees latitude. However, plants with a hardiness of up to 8b will grow in Glasgow, whereas Moscow, with a continental climate, it is just 5a.
The coldest zone in the British Isles is the Scottish Highlands, which is 7a. However, again this is still milder than places such as Warsaw, Poland which is zone 6b.
The very tip of Cornwall, Scilly Isles and South West Ireland is 10a. This is the same classification as Palma, Mallorca and Nicosia, the capital of Cyprus! This means that gardens in these areas can grow palms and plants that are more usually seen on Mediterranean holidays.