Pre-brexit it was easy to visit England/Scotland/Wales via ferry (though technically illegal) but these days they've really toughened up on this and I don't recommend trying it if someone was to recommend it. Anyone non-white tends to be picked out and asked for identification now.
If you check your government's website you might be able to view a PDF of the treaty signed between Belgium and Thailand. I was able to download a PDF of the agreement between Ireland and Thailand which detailed everything that is covered and answered the questions I had.