For moving personal effects from abroad to UK, both non-citizen or citizen UK need to declare the TOR form before proceeding the shipment.
Here is the link and guidelines to fill TOR form : https://www.gov.uk/guidance/application-for-transfer-of-residence-relief-tor1

After filling the TOR, it will take around 10 – 15 days by UK government to submit and give back the confirmation with UN number.
Then customer will give UN number to the logistics company who is responsible for shipping their personal belongings.

Example of the confirmation of TOR with UN number :