In case the United Kingdom leaves the European Union without agreement on January 31, 2020, changes will happen to its borders. This means the end of free movement in all countries and the reinforcement of security at all borders and airports.
At airport level, the blue customs channels (from European Union) will disappear, and blue passports will be introduced in the UK at the end of 2019.
For now, border crossing agreements will be maintained. Citizens of the European Economic Area (EEA) will continue to enter the United Kingdom using their country's passport or ID card. Passengers can use the e-Gates (turnstile with a passport chip reader) when traveling with a biometric passport without routine issues upon arrival.
EEA citizens are still subject to security checks as usual.
From that date, passengers who have nothing to declare should take the green channel. Passengers who have something to declare should go through the red channel.
The UK's official statement can be found here:
For more information, see also the following pages of the British Government: