. 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.
Aah. Yes. Just tell them as best as you can it's very urgent and you need the TM30 completed to stay in Thailand or alternatively go to Immigration and ask them to call the Landlord. They can face fines upto 500 Baht a day for not registering you as living there
Maybe the place where you are staying doesn't understand the TM30 process. Can you ask a Thai to ask them to tell them the issue you have then they can go to Immigration and register you as living there (TM30)