You can still get them for even less if you buy from a seller in China directly and Brexit has no affect on purchasing from China, just the EU.
Indeed. If anything, members of the EU are now worse off regarding buying outside of the EU, in particular for low amounts, since VAT is now systematically applied to any import in the EU (only since a few weeks I think) regardless of the amount, whereas before, there was a threshold (which I don't quite remember as it has changed a few times.) But certainly, if you were buying for a few euros of goods, for instance from China, you weren't charged VAT. Now, you will be.
I dunno how the UK handles this, but if anything has changed, it has nothing to do with the Brexit. Unless indeed you buy from some country itself in the EU, but as someone said above, why would you bother buying $3 boards from an EU country, if you can even find them? Not sure I get it.
In any case, as I said, definitely consider the RPi Pico in this price range. It's a british product, and it's distributed within the UK.
https://thepihut.com/collections/raspberry-piThe Pi Hut is an official reseller and operates in the UK.