There have been recent UK Government announcements relating to deadline extensions for the start of plant health inspection regimes (known as sanitary and phytosanitary checks), but these do not apply to Wood Packaging Materials. Inspections have already started and are continuing.
UK Defra (Department for Environment, Food & Rural Affairs) have issued the following statement:
Since 1st January 2021, all wood packaging material (WPM) moving between Great Britain and the EU, and from Great Britain to Northern Ireland, has needed to be treated and appropriately marked in compliance with international standards (ISPM 15). Defra can confirm that this remains the case: all WPM moving between Great Britain and the EU, and from Great Britain to Northern Ireland, must be treated and marked in compliance with ISPM 15. This is in line with international requirements for trade and is in place to protect both the EU and UK from potential harmful plant pests and diseases." UK Government, Defra, 17th March 2021.
(link takes you to the UK Brexit web page)