Car insurance if UK resident but often in Belgium

If you are permanently resident in the UK, then you need UK based insurance. Make sure you declare that you spend a lot of time abroad.
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If you then move to Belgium, best thing to do is to make sure you have 3 months cover to go on your UK insurance, and import the car to Belgium.
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You can get temporary car insurance based on a VIN number if your UK insurance has expired and you've not yet completed the Belgian registration/import process.
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Cost is based on your previous claims and the model of the car - not how long you've been driving or the age of the car.