You need to submit a police clearance certificate now.
If you're a UK citizen and don't have formal residence or citizenship in another country this is getting a legalised ACRO.
You need to get a hard copy ACRO
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Then before the Thai MFA will accept it you need to get that legalised by the UK FCDO (or an approved Apostile service)
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Takes longer and costs more than your DTV application and approval does.