You should check your customs channel status as below for Stena Line routes:
UK Exports – prior to arriving for check-in at Holyhead or Fishguard (or Belfast if your movement from NI to GB requires the use of a GMR). A GMR is not required for UK exports via other Stena Line port locations (i.e. GB TO Netherlands where GVMS is not in use, or GB TO NI where no export declarations are required).
UK Imports (on Netherlands to GB routes, GB to NI routes and for transit movements from Republic of Ireland to GB only until further notice; GMRs are not currently required for standard imports via Ireland to GB crossings) – during crossing, from 10-20 minutes prior to arrival at UK port.
Please note – where a GMR is required, it is your responsibility to check if an inspection (export or import) is required and to act upon that information. Stena Line are not responsible for the information held or the accuracy of the information on these external government websites and shall have no liability if you do not access these links or fail to act upon the information provided. The websites can also be accessed directly at https://www.tax.service.gov.uk/driver-inspection-notification/start in the event that this website, or the above links, are unavailable.