This may be, but the question is why they refuse to ship to the UK.
If the custom forms are correctly filled, it shouldn’t be stopped by customs. It may need a bit more work, but if 50% of the EU sellers can do it, it is a bit surprising the other 50% of EU sellers can’t do it. I don’t really know how their situation would be different.
To be honest, I am mostly annoyed because about 9 of the 10 times I asked the seller if any arrangement could be made to ship to my address, making it clear it was a rare item I couldn’t simply buy elsewhere, the seller did not respond at all.