Tracked is always pretty high. If you use First Class International to the UK, it will probably be tracked all the way. However, I think it will stop at the US border when you ship to Thailand (as it does with most countries).
We use https://www.17track.net/en to see the tracking info as it usually shows both USPS and info for the destination country.
You can also send untracked or International first class, but build-in an extra margin to the international shipping cost in case of postal losses. Some countries/cities are more reliable than others for untracked mail.