This subject has been covered in a number of other modelling forums very extensively especially Britmodeller.
HMRC have introduced a scheme where the VAT should go to the country the item ends up rather than where sold and this system has no lower level threshold, only an upper figure of £135 where the procedure changes. If the seller abroad complies with the HMRC rules and pays the VAT to HMRC then the order should pass through Customs without further charge but it is true to say that plenty of small businesses abroad are not prepared to dance to the HMRC tune and are either refusing to take UK orders or send them without doing anything about the VAT which risks them being returned or being taxed anyway with a substantial handling fee added on. To judge from Britmodeller postings, there seems to be a "wave through" policy at UK Customs for most small packages perhaps while the aftermath of Brexit settles down a bit but presumably this will change soon.
As I understand it, other countries and particularly the EU are going to apply the same procedure in the near future so it is not really right to say that the situation is a result of Brexit, rather something that has been brewing for a while mainly because of purchases from the Far East flouting the previous rules on a large scale. Hopefully things will eventually quieten down but it is still a case of watch this space at the moment.