I'm in the same position. There is no CGT in Australia for non-residents when the gain is made on Non-Taxable Australian property, and Singapore, where there is no CGT at all. The income has been reported in the country it was made, but is legally tax free there, so definitely not tax evasion. In the past this was easy to remit to Thailand without having to pay any tax on it here, by using the "earned in previous year(s)" method. Unless / until Thailand starts taxing all overseas income, regardless of whether it's remitted here or not, the capital gains you make in such countries are not taxable here, as long as they're not remitted. Now, at this point I was going to agree with you on using the gain from one asset to buy another, and then sell that at break even, or a small loss before remitting it to Thailand, but while thinking about how to word it I changed my mind, as I suspect the TRD will only be interested in the sale of the second asset as a whole. Unless you realised the initial gain, and bought the second asset prior to Jan 1st this year, I think it will all be assessable. I hope I'm wrong, but everything I've read in the tax code, and talking to TRD people, I fear that I'm right.
Using a similar example, If I made a capital gain of $10,000 this year and put it in a bank, then remit the full amount next year, I'd need to report that as assessable income. Even if bank fees caused that amount to drop slightly, so I'd made no gain on that money, it would still be assessable. Buying shares may be treated differently, but I suspect it will be the same. The TRD will see that you sold £10,000 of shares, that you didn't hold prior to Jan 1st this year (if that even matters, but that's a different can of worms), and will want to collect the tax on it. In any case, this is definitely a question for a reputable tax advisor. (However, I'm holding off asking any such questions until the tax advisors themselves know what is going on. Hopefully later this year, though given that this year's tax situation regarding remitted funds is no different from previous ones, in that income earned prior to this year is still non-assessable, it may not be till late 2025 that things actually become clear).