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Source: UK Government

Veterinary Surgeons should obtain an import certificate in advance if you think you will need to import/use an imported medicine during this period.
In urgent cases where the VMD have previously issued a certificate, you may purchase and use an imported veterinary medicine prior to obtaining a certificate from the VMD. This is a special dispensation which only applies to the VMD online system during this maintenance period.
You should obtain an import certificate retrospectively as soon as possible.

MIL OSI United Kingdom