ONS:
"The source for data on trade in goods with non-EU countries is Extrastat. In general, the figures for trade with non-EU countries show the trade as declared by importers and exporters or their agents and for which documentation had been received and processed by HMRC during the month.Under the procedures for control of exports, the principle is the same–namely that goods cannot be cleared for export until a Customs declaration has been made."
"Traders can, if they wish, submit a simplified declaration so that the goods can be exported, which has to be followed within 14 days after date of shipment with a complete export declaration.The processing of these complete export documents begin three working days before the end of the calendar month (two days for December)." "Thus, the export statistics compiled for a month (which are based on the date of receipt of the complete export documents) do not correspond with goods actually shipped in the calendar month."
"In general, the figures for trade with non-EU countries show the trade as declared by importers and exporters or their agents and for which documentation has been received and processed by HMRC during the month."