You cannot assume the regulations laid down by the Canadian Govt or the US Govt are the same as those used by HM Customs
The Guide to importing and exporting issued by HM Customs clearly quotes "Who is this publication aimed at?
This information pack is for anybody, whether already in business or not,
It then goes on to refer to "When you import goods, you must declare a value (known as the customs value) on the C88. It is important that this value is correct as this is the amount on which any duties and VAT due will be calculated. The customs value is also crucial to obtaining accurate trade statistics."
So whether you are importing one item or several the importer is subject to the same taxes/duty.
"How do I determine the customs value of my goods?
Where the goods you are importing are subject to a sale, the customs value should be based on the CIF price (cost, insurance, freight) plus certain other costs you may have incurred in purchasing the goods (e.g. some commissions, royalty and licence fees."
So your phrase" a single import duty scheme" has hit it right on the head, because that is exactly what it is
That was fun. Are you finished?
In your glossing over of the HM Importing/Exporting Guide, you missed several points which are pertinent to the discussion here. Specifically, it would seem, you decided not to share with our audience the terms of valuation; specifically, that valuation occurs based on
transaction value. - i.e., when a distributor in the U.K., does not want to pay massive VAT and duty, they may take the shipment on consignment. As no transaction has taken place, the valuation process is altered, and duties are
significantly lowered, or even relieved completely. In instances of consignment transactions $4000<, no valuation declaration needs to be presented at all. Believe it or not, but consignment is an incredibly common practice, for just this reason.
Now, in the circumstance that consignment is not undertaken, the distributor (and they will all do this) will hire a professional Commercial Brokerage Firm to negotiate tariffs and/or VAT and/or duty fee reductions. This is a necessary practice given shipments of the magnitude we are talking about in this particular discussion. There are several methods of VAT reduction - or complete relief - though it is not necessary to describe them all here. Suffice to say, my comments about you lacking scope and scale are applicable.
Further, it would seem you declined to to research which goods from which countries levy which particular VAT and/or duty and/or tariff fees. So, as I said earlier, the massively increased price from U.K., retailers is
not due to VAT and/or duty fees.