The UK has consistently averaged around 700,000 tests per day for the last 6 months. Admittedly some of these may be pro-active screening tests for healthcare workers et al, so let's say 500,000 is general testing. This is still 10x the amount that Thailand is currently doing and you don't have to be Heinz Wolff to understand the simple laws of statistics in that if Thailand was to increase its testing 10x, it would most probably find near 8-10x the amount of positive cases?
The UK, US, EU or any other similar countries aren't stupid and cannot be fooled on this one due to their experience of the extensive testing that they have carried out. They are simply not going to risk their economy, the health of their people and possible civil unrest due to further lockdowns, on the word of any official from any country that they feel is not doing enough to control and monitor the pandemic. The sooner Thailand learns this and acts accordingly, the sooner they will be off that red list.