A product can claim to be 'made in the UK' without any of it being manufactured here, the country of origin is where the commodity code last changed. After 'Made in UK' there should be a breakdown of what % of components (over 20% in value) come from which countries. i.e. China 56%, India 25% etc.
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To aid the UK economy and for ethical reasons, many people wish to support UK industry and buy British, but greater transparency is needed.
The country of origin is where the commodity code last changed, so if you wished to buy a greenhouse which is basically just glass and aluminium, the glass could come from India and has a commodity code of X, the extrusions could be Chinese with have a code of Y, but when the 2 boxes are put together if has a commodity code of Z and is 'made in the UK'.