All currency values imported into your ShipStation account are treated as the native currency of your account's home country. So, if your account is based in the UK, all monetary values imported into ShipStation from your selling channels & carriers, or the values displayed on your customs values, are treated as being transmitted to ShipStation in GBP amounts.
Keep in mind, ShipStation will not do any currency conversion of values importing into ShipStation. The values are anticipated to be in the native currency value of your account's origin country. Any conversion will need to be done prior to the orders importing into ShipStation.
Using the native currency value will also best ensure that the shipping charges, and customs fees & taxes are being paid at the appropriate rates!
For any additional support on this, reach out to ShipStation support!