You would need to de-register from VAT if your Limited Company goes into administration, gets de-registered, or goes dormant. You would also want to de-register from VAT if your earnings fell below the threshold for 12 months and you believe that they would continue to do so in the immediate future or you stop making supplies or relevant acquisitions. If you sell your business you can either allow the new owner to continue using the VAT number registered to the business but you would need to still de-register yourself from that number.
To de-register from VAT you would need to complete a VAT7 form, which can be done online, requesting to cancel your VAT registration. At Taxfile we can do the de-registration for you and take out the complexity of the administrative requirements in de-registering you/your business from VAT.