Currently the UK is negotiating a Withdrawal Agreement with the EU that needs to be ratified by the UK, i.e. it needs to be voted for in Parliament. If the Withdrawal Agreement is ratified before the 30th March 2019, then EU law will cease to apply to and in UK on 1 January 2021, i.e. after a transition period of 21 months, with a phased approach to changes over the transition period.
Information regarding changes at the UK border in the event of a no-deal Brexitwww.gov.uk/government/publications/partnership-pack-preparing-for-a-no-deal-eu-exit
Mayor of London Brexit resource hub: www.london.gov.uk/press-releases/mayoral/mayor-backs-business-with-new-brexit-hub & www.growthhub.london/brexithub/