There can be huge delays at HWDC. It's Royal Mail international hub and when parcel volume is high like during Christmas, parcels take longer time to be processed. It will eventually be released. All I can advise is to be patient.
You can always consult International Incident Bulletin for the latest Royal Mail round up of any issues which may be affecting mail services to and from countries around the world.
For example as of August 31, 2021: Items to some EU destinations are also experiencing customs delays. This is the result of changes to the EU’s VAT rules for items from non-EU destinations from 1st July. Also delays should be expected as a result of ongoing COVID-19 reduction measures and reduced air transport capacity.
Heathrow Worldwide Distribution Centre (HWDC) is a sorting office for inbound and outbound international mail operated by Royal Mail. Located close to Heathrow Airport, the HWDC is situated in the town of Langley, Berkshire.
HWDC is one of the most automated postal centres in Europe. Automated conveyor systems transport trays of mail to automated mail processing equipment and manual or special handling workstations. The conveyors read the bar code labels on the trays and then transport them to the desired destination. When the mail is sorted on the automated mail sorting equipment, the letter destination image is lifted by high speed optical cameras. Both inbound and outbound mail are then automatically sorted into groups.