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://developer.postnl.nl/browse-apis/send-and-track/products/#destination-rest-of-world

Go to this page for documentation for generating the required S10 barcodes:

https://developer.postnl.nl/browse-apis/send-and-track/barcode-webservice/documentation-soap/

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:

https://developer.postnl.nl/browse-apis/send-and-track/labelling-webservice/documentation/

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.