Well, the day has come, first watch stopped. Actually not seized, but stopped for customs clearance.
I ordered a watch from Sead's UK stock, since Brexit a few of them have arrived without problems, but today I received a letter from Polish Post Office asking me to present documents for customs clearance of a package from UK.
It seems that they did not open or seize the package but just flagged it for normal customs clearance procedure. I am not going to answer I think and it will go back to UK, but it is the first time this happened and I don't think I will order from UK again. I think we had a few months of easy treatment and now the procedures are the same as for any other country outside EU.