not sure what you mean here but anything you made in the UK that you didn’t pay tax, and then bring those fund untoThailand would be taxable. Chances of them trying to grab it is another thing.
pretty simple. Just know the difference between pre and post tax funds. If you have already paid tax on the money coming into Thailand then its non taxable. Once the money is in the Thai bank and you buy stock or something you buy and then sell any profit (after expenses) would be taxable).