Updated : 12/04/2017
For the time being, the United Kingdom remains a full member of the EU and rights and obligations continue to fully apply in and to the UK:
If you are a non-EU national wishing to visit or travel within the EU, you will need a passport:
and possibly a visa. Apply for a visa from the consulate or embassy of the country you are visiting. If your visa is from a "Schengen area" country, it automatically allows you to travel to the other Schengen countries as well. If you have a valid residence permit from one of those Schengen countries, it is equivalent to a visa. You may need a national visa to visit non-Schengen countries.
Border officials in EU countries may ask for other supporting documents such as an invitation letter, proof of lodging, return or round-trip ticket. For the precise requirements contact the local consular services of the EU country in question.
There are a number of countries whose nationals do not need a visa to visit the EU for three months or less. The list of countries whose nationals require visas to travel to the United Kingdom or Ireland differs slightly from other EU countries.
Entry conditions to the United Kingdom or Ireland are based on their respective national law and differ from the EU rules.
Do not forget your travel, health and/or car insurance documents.