XE.com's rates are the most accurate technically, but unless you have a credit card that has an equally accurate £ to $ conversion rate and they don't charge a small percentage for handling $ transactions, then the XE.com figure is never an accurate representation of what you pay Grim, it will always give you better rates than what you'll actually pay. Amazon.com's rates too are never as good as XE.com, that's why you think you're paying less when you go for the $ option at amazon & calculate the £ cost at XE, but in reality your bank still has to process that $ order and it may have worse conversion rates than Amazon, so in the end it may have been better to choose the £ option at amazon anyway. (I know this is getting confusing, it's hard to explain!

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Anyway, regarding the customs rate, you have to estimate when customs will have the item in front of them, not when the product shipped. For instance if you place an order on August 26th and it ships on the 27th, it might be September by the time customs have the product, so you actually have to apply the September conversion rates, not the August.