Does Great Britain use celsius or Fahrenheit?

The United Kingdom is officially a metric nation, but retains considerable use of older imperial units. So, Celsius is now the main (and official) system of measurement, but some older people will still find Fahrenheit more familiar.

More specifically:

Officially the UK is metric, so Celsius is used for the purposes of law, regulation, trade and public administration.

The Metereological Office converted weather forecasts to Celsius in 1962. Conversions to Fahrenheit were given at first, but are now rarely given in the broadcast media. One exception is in the newspapers, some of whom occasionally report particularly high summer temperatures in Fahrenheit.

In the kitchen, cooking appliances feature temperature controls in Celsius and have done from the 1970s onwards. Cooking instructions on packaged food is primarily in Celsius, as are published recipes, although Fahrenheit equivalents are sometimes provided for people using older ovens.

Heating appliances and controls are usually marked in Celsius, except in rare cases such as where an item has been imported from the USA. Thermometers can be bought with dual scales, although Celsius-only versions are seen too; Fahrenheit-only displays are exceptionally rare.