What is a visa?

A visa is a certificate that is put into your passport or travel document by an Entry Clearance Officer at a British mission overseas. The visa gives you permission to enter the UK. In certain circumstances you must have a visa before you arrive in the UK – see below Do I need a visa?
If you have a valid UK visa, we will not normally refuse you entry to the UK unless your circumstances have changed, or you gave false information or did not tell us important facts when you applied for your visa.
When you arrive in the UK, an Immigration Officer may ask you questions, so take all relevant documents in your hand luggage. This may include a printed version of your return flight itinerary or evidence of funds, for example.