Pet Movers to UK

"cheaper alternative"
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Take the cat yourself?
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P&O allow pets travelling in a car, or carried by a foot passenger.
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http://www.poferries.com/tourist/content/pages/template/_footer_About_FA...
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If you are travelling by car then your pet will need to stay in the car for the duration of the crossing. If you are travelling by foot, then your pet will need to be in a lockable pet carrier of sound construction, which is able to withstand being secured on the ship. You must be able to carry this container onboard yourself. Pets are then kept in a secure area on the car deck for the duration of the crossing and are regularly checked by crew members. The latest check-in time for foot passengers is 45 minutes, however it is advised that extra time should be given if a pet is being transported. There is a charge of £15.00 each way per pet.
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By car, if you did it as a day trip, the cost probably wouldn't be much more than €300, ferries and petrol costs included.