As has been said, this has been discussed to death but here is a quick breakdown.
1/ VAT 20%
2/ Import duty 5%
3/ EU/UK enforced warranty obligations. For example I believe most EU countries have a 24 month warranty, the USA get 12 months, this is a real thing that costs Canon Europe money Canon USA doesn't have to spend.
4/ Economies of scale.
So take a price of $3,899, put 5% duty on that, $4094; put 20% VAT on that because VAT gets charged on the duty too, $4,913; throw in an extra 12month warranty for a total of $5013. Or £3,972. Now adjust for the economies of scale because the USA market is a lot bigger than the UK market so it is a lot more efficient and you are barking up the wrong tree. Get over it or get on a plane and get one from B&H, it will cost you $3,899 plus sales tax, or $4,245. You can claim the sales tax back but you are then liable to pay the UK duty and VAT and you are still out the price of the flight and accommodation and should you need warranty work you will probably have to ship it back to the USA.