Small Parcel Reference Number (SPRN) is the VAT number handled by Her Majesty's Revenue and Customs (HMRC). Some also refer to it as the HMRC number.

HMRC is the tax authority of the U.K. government and will therefore handle all customs and duties on import and export of goods to the UK.

Contact HMRC to get registered
https://www.gov.uk/government/organisations/hm-revenue-customs/contact/vat-enquiries 

Important VAT Rules

VAT is required on all goods sent from:

• outside the UK to Great Britain
• outside the UK and the EU to Northern Ireland

Exceptions

• VAT is not required if the value of the goods is below £135
• VAT is not required on gifts with a value below £39

Read more: https://www.gov.uk/goods-sent-from-abroad/tax-and-duty 

Add your SPRN / HMRC number in Webshipper

To add your SPRN / HMRC number to your sender information,

Go to Settings > Locations > Customs numbers, select the type you wish to register, and enter the number. Finally, click Add and the number is automatically added to your shipping info on all UK shipments. 

You can find it on your customs documents and on labels.