Freezing on the ferry to Santander

Am taking the motorhome on the Plymouth-Santander ferry. Have been told that Health and Safety issues mean that Brittanny Ferries won't let me run the Thetford gas fired fridge freezer during the voyage. (It seems refrigerated lorries are provided with electrical plug ins but this privilege / courtesy is not extended to motorhomes and caravans). Thetford were very helpful and suggested taking ice packs and boosting the fridge temperature downwards immediately prior to sailing. Can't believe we are the first CC members to face this issue so thought I'd post on this forum and ask for advice for which we would be grateful.