Staying 180 days in a year means you become a tax resident in Thailand. You have to submit a tax return showing income you brought into Thailand. (A proposal not yet passed into law is that you would have to declare all income globally, even if you didn't bring it to Thailand). You assess income tax according to Thai law to determine tax payable. Then deduct tax paid in your home country to determine if any tax is due in Thailand.
Remember it's cumulative days in the year, Lee. If you leave on day 179 of (say) 2025 and returned on 31 December, you would become tax resident the moment you enter Thailand
Indeed they do. Most countries protected their citizens' pensions from the risk of additional taxation, but the UK only protected government pensions 🤨