The present Brexit deal states that a transition period applies immediately after the withdrawal. You may still consider the United Kingdom a member of the EU during this period. In keeping with this policy, you must declare your trade with the UK as part of your Intrastat declarations, just as what was customary in the past.
The transition period is in place at least until 31 December 2020, but may be extended once by up to two years. We will be informing you on any changes once these apply to your Intrastat declaration.
We hope that we have provided you with sufficient information.