I'm not an expert on regulatory matters but normally whether the UK is in or out of the EU, or now versus sometime before, doesn't make a difference in terms of plant imports. Right now you can still order from the EU without paperwork, but from everywhere else, paperwork of some sort has always been needed, and maybe the authorities simply became more stringent about it. A quick online search indicated that at minimum (and this is from 2011), the UK required that you arrange an import permit, a CITES export permit if applicable, and a phytosanitary certificate. (see first post here:
Importing orchids to the UK - how to help prevent extinction, and you can find countless other sources saying the same). Flasked orchids are a bit different, and I believe that less is needed, but at minimum you need an import permit. If you have ordered plants from Thailand before without doing this, it was completely illegal. The seller in this case is correct in saying he can't ship to you.
Importing from abroad has always involved arranging a varying number of permits, and it's actually a good thing. These permits help ensure that plants are coming from cultivated sources rather than wild collected (CITES), which is a huge, huge problem for countless orchids. Controling what is imported also helps prevent the spread of pests and diseases.
Quite honestly, as a new-ish orchid grower (if I recall correctly from one of your previous posts) you can find a huge variety of orchids to buy from growers within Europe, and the plants will generally arrive in far better health that plants that have traveled around the world. What orchid were you looking to buy? Perhaps I or others here can help you find a local, legal source.