On March 19th, the UK published a first draft of a withdrawal agreement, which although not finalized, indicates that for value-added and excise taxes, much of the status-quo with the European Union (EU) will remain until January 1, 2021.
Notably, the Draft Agreement states that the UK and the EU will agree to have the EU Directive on VAT (Council Directive 2006/112/EC) remain as the controlling doctrine for matters related to VAT, when goods are transported between the UK and the EU. This agreement would remain in effect during the transitional period, which would extend until December 31, 2020. Other provisions of the Draft Agreement address excise taxes, citizens' rights and customs procedures. The Draft Agreement has been published by the UK government here.