Once the Brexit takes effect it will not be possible to ship to the UK with an EDI/FTP connection (Zelfbouw). Parcels Non-EU - the shipment method for shipments outside the EU - is simply not available on the EDI/SFTP “zelfbouw” platform. If you wish to continue shipping to the UK with PostNL you will need to implement EMS combilabel shipments via the API. Documentation for the Parcel Non-EU (Parcels Non-EU) implementation can be found here:
https://docs.api.postnl.nl/#tag/Product-codes
Go to this page for documentation for generating the required S10 barcodes.
Check the following documentation for information about generating labels and confirming shipments for Parcel Non-EU (Parcels Non-EU). The customs type section contains the specific fields for a Parcel Non-EU shipment.
For incidental shipments to the UK you can also choose to use the MijnPostNL environment. You probably already have an existing account or you create a new account via mijn.postnl.nl.