In a June 23, 2016 referendum, the U.K. voted to leave the European Union, making it the first EU country to do so. The event became known as Brexit, short for British exit. David Cameron, the then prime minister, resigned the next day. Theresa May, who replaced Cameron, tried three times to negotiate a deal with the EU, but failed on all accounts. The former mayor of London, Boris Johnson, is now prime minister and a Brexit supporter.
As of now, the U.K. remains in the EU, due to multiple extensions. Once the U.K. leaves the EU, and depending on the deal if leaves with, it will no longer be a part of the customs union and single market. Being outside of the EU will lead to increased commerce cost and transit time between the U.K. and EU countries.