ToR form

A Transfer of Residence (ToR) form is where you tell the UK government all the stuff you’re bringing with you into the UK. Normally, you have to pay customs fees when you import goods into another country, because usually you’re intending to sell them there. Luckily, there’s an exception for people who are bringing their household goods with them because they’re moving to another country. The ToR form is what UK customs will use to decide whether to charge you import duty on the stuff you’re bringing in with you.

Find the ToR form here:  

Application for transfer of residence relief (ToR1)

Use this form to apply for transfer of residence (ToR) relief when moving or returning to the UK.

https://www.gov.uk/guidance/application-for-transfer-of-residence-relief-tor1

You will also need to submit an inventory of what you're bringing with you. This doesn't have to be overly detailed: "clothes" is a sufficient level of detail; you don't need to list every shirt and sock!