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The VFS site states that using the Priority Visa service does not guarantee that your application will be successful. All visa applicants must meet the requirements of the UK Immigration Rules. If your application is not straightforward for any reason, it may not be possible to meet the priority timescale https://www.vfsglobal.co.uk/th/en/premium-services/priority-visa The priority fee is non-refundable if an application is unsuccessful or takes longer to process.