Effective January 01, 2021 the United Kingdom and certain associated territories will no longer be covered by the Canada-EU Comprehensive Economic and Trade Agreement (CEUT) by virtue of Brexit.  This means that imported goods must now be accounted for under the Most-Favoured-Nation (MFN) tariff treatment.
Importers of goods originating from the United Kingdom, Channel Islands, Gibraltar and Isle of Man, that are eligible for the CEUT preferential tariff treatment and that are in-transit to Canada before January 1, 2021, may claim the CEUT preferential tariff treatment.
Bill C-18, An Act to implement the Canada-United Kingdom Trade Continuity Agreement (CUKTCA), was introduced in the House of Commons, however has yet to be passed.
Until such time the CUKTCA is passed, the the Government of Canada has issued the United Kingdom Trade Continuity Remission Order (20-1135) that will remit the difference between the Most-Favoured-Nation Tariff rate of customs duty and the rate of customs duty that would apply under the Comprehensive Economic and Trade Agreement (CETA).
Importers of qualifying goods may cite the remission order (20-1135), in order to benefit from the remission of duties.
Field 14 (Tariff Treatment) of the B3 Canada Customs Coding Form should indicate Most-Favoured-Nation (MFN) tariff treatment 02.
Field 16 (Special Authority) of the B3 Canada Customs Coding Form should indicate 20-1135.
Implementation of the United Kingdom Trade Continuity Remission Order, 2021 – Customs Notice 20-39
Text of the Canada-United Kingdom Trade Continuity Agreement