UPDATE : HMRC has issued further easements (imports to UK) – here.
The following are latest instructions issued by the UK government and agencies :
(2) using UKCA label (goods) – and CE marked goods circulation – here.
(3) letters issued individually to EU trading businesses – here.
(4) simplified customs procedures (updated) – here. (I updated the post I had made earlier on this topic)
Transitional simplified procedures – here.