Integration with Shiptheory for the UK. Shiptheory integrates with all of the local providers.
https://shiptheory.com/Shipstation doesn't have any solid plans to add UK providers.
Mark from Shiptheory here.
Just FYI, Dearsystems have added Shiptheory as an integration now.First create an account here @ https://shiptheory.com
Then while logged into Dearsystems, navigate to here: https://inventory.dearsystems.com/Shiptheory