Ok. Let’s break this down - there’s quite a few things that someone buying goods overseas needs to be aware of...
Import duty and tax has always been applicable to goods coming to the UK from outside of the EU - your items from Russia, Israel etc are nothing to do with Brexit.
Firstly, you should’ve established your incoterms with the seller - these are rules that stipulate who is responsible for paying for the various elements in the transport chain - a quick google of incoterms 2020 will give you full information on these. Looking at the info you have provided, it sounds like this has been processed EXW - that puts all of the responsibility on you as the buyer to pick up the charges for everything.
Now the goods have arrived in the UK, let’s take a look at the clearance costs...
In order to calculate the duty and tax costs you need to have a look at the goods’ HS/commodity code in the UK customs tariff - as part of the Brexit agreement, the vast majority of items are zero rated for duty for importing from the EU, but duty rates will vary for other 3rd country locations.
VAT should be chargeable at standard rate from everywhere in the world - although some countries do have preferential rates, again, you need to refer to the UK’s customs tariff to check.
If you are VAT registered, then this is reclaimable from HMRC as part of your tax returns processes.
Import duty is calculated against your CIF cost, so it’s a percentage of the Cost of the goods, Insurance and your Freight costs - it is not just based upon the sale cost of the goods.
VAT is calculated on your CIFD value - all of the elements for CIF as above + any duty paid as well.
All of this information is available through a quick google search, or read of the .gov website.
The vast majority of customs hold ups are caused by people not doing basic research into what’s exactly involved. Unfortunately, there appears to be an element of this here - the fact is, these fees aren’t really negotiable as they are governed by the import tariff, and there’s no way around paying them.
Source: 15yrs in export and import industry.
Best of luck to you with future transactions - hopefully the above insight is helpful.