I will have a go answering this - tax and import laws in the UK, like the US, are extremely complicated and often bizarre.
It's therefore almost certain nobody except Raspberry Pi's accountant / legal advisors would be able to give the definitive reason, but I am pretty sure it is to do with how components vs full computers are classified for duty purposes.
There are specific import laws designed to aid manufacturing industries, and it could be that these laws need to be extended to component assembly. For example, the UK does very well in car and aerospace manufacturer, and there are import laws regarding components designed to aid this.
It's therefore almost certain nobody except Raspberry Pi's accountant / legal advisors would be able to give the definitive reason, but I am pretty sure it is to do with how components vs full computers are classified for duty purposes.
There are specific import laws designed to aid manufacturing industries, and it could be that these laws need to be extended to component assembly. For example, the UK does very well in car and aerospace manufacturer, and there are import laws regarding components designed to aid this.