If the UK agrees a deal with the EU before 29th March 2019, then the UK is expected to enter into a transition period. This period is expected to remain in place until 31 December 2020, subject to a possible extension.
The transition period is designed to give everyone in the UK and the EU time to prepare for changes caused by the UK leaving the EU and the customs union at the end of 2020. As a result, the way the UK trades with the EU will remain broadly the same during the transition period. This means that you won’t have to make any immediate changes to the way you think about customs or VAT in relation to the EU. You will need to start preparing for the changes during the transition period which must be completed before the end of 2020.
To find out more, go to the Government’s official