The need for special permissions to visit Koh Samui depends on current COVID-19 regulations and personal circumstances, like your place of origin. If you are already in Thailand, it appears that no special permissions are required, and you can travel there either by air or by taking a combination of land transport to Surat Thani and then a ferry. However, travelers from certain countries may face restrictions.
Just had a friend go to visit Samui. Had a video chat with him while he was there. Absolutely deserted. Many places closed. Beach had 3 dogs and 1 person. Depends if you are looking for peace and quiet. He lives in Thailand and had no problem getting there.
If memory serves me right you're already in Thailand, yes? If so, then no permission needed - just either spend a fortune on a direct flight or fly/train to Surat Thani and get a boat across.
No brits allowed for what I've been told...however, thai government....burp , the army will ultimately decide .I reckon back end of summer if your lucky
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