Yes. It's technically an Overstay. However, in the majority they do nothing about it as it's not a working day for the Immigration Centres. Most of them will not even stamp your passport either.
. It is true. If the date expires over the weekend you can on the next available working day and will not be charged. The issue you have is this could be dependent on the local rules of the specific immigration centre. But generally speaking you are not charged
. Check in Heathrow with your UK Passport. Check in BKK with your Thai Passport. Check Out Thailand with your Thai Passport, Check In to the U.K. with the U.K. Passport. Really is that easy.
As a Director or Shareholder of a Thai Limited Company as long as he doesn't work in the Company he does not need a Work Permit. He could in theory just opt for a "Normal" Visa. Many investors do this already.
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