The current VAT rate is 17.5%, close to the 20% you are seeing, so if that £457 you quote includes VAT (and the $ price also includes any relevant sales taxes) it would explain a large par of the difference.
Other reasons for US/UK price differences:
Exchange rate fluctuations. Next week the changing relationship between the pound and the dollar might make the comparison close, or might swing the other way and make the UK price seem even more expensive. Companies that work predominantly in one currency may pad their product prices in other currencies to allow for this variation, so a drop in a currency's worth doesn't eat too far into their margins (or they may just sell in that one currency and let their customers, the retailers, deal with the variation - the retails are likely to deal with the possible variation by padding their margins a little too).
Distribution costs, including import duties, and "economies of scale" - differences in market size and the cost to transporting items into the UK compared to the US could be part of the difference. If the company pays more to get their good to the local market then they will pass that cost onto the retailer who will pass it on to you.
"What the market will bear". If companies can get away with charging extra, they will. Because they UK companies don't compete directly with US companies for most of their trade the price US retailers are charging usually does not have a significant direct impact on what price customers in the UK market will be willing to bear.
As for whether the US sourced CPU will work in the UK: it will, as motherboards and CPUs do not vary between countries (your PSU deals with translating your local power supply to the DC voltage and current requirements) and there is no other region locking on these parts either. But you might find any warranty you get on the part is not valid over here so there is that risk to consider. Also be aware that you might be charged import duty on the CPU - $500 is enough for customs to be interested, assuming it is properly declared of course.