This additional "manufacturer's" warranty is little more than another attempt by the camera manufacturers to keep the prices of their products at an artificially high level in the UK.
If you buy an item from an official retailer in the UK you normally will get a 1 year warranty. You also have additional cover under the Sale of Goods Act (NO ! it isn't a 6 year warranty - and there isn't a "secret" EU Law giving a two year warranty

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If an item is going to fail it is highly likely to fail within its first year - usually very early on.
By Demonising "grey imports", and refusing to honour additional warranties on them, the manufacturers are in effect trying to make the customer pay their inflated prices for an extra year's warranty - which they are very unlikely to be called upon to honour.
If you believe that the "grey importer" is a reputable UK based business, I see no reason why you shouldn't deal with them - if you are nervous, you can always consider using some of your saving to buy an insurance/warranty from a third party.
It might even be worth doing this with an imported item where exercising your legal rights on a failed item may be difficult.
Really: you pays your money............................