How can you add extra s&h and sg&a? This stuff goes from Asia, you ship it either to US or to UK (or whereever). You don't ship it from Asia to US and then to UK. The shipping should be about the same to US and UK.
Similarly with sg&a. The US operations does not have it already included in price? Or the UK (EU, whatever) customers pay for that twice?
I would imagine that that £91.56 price difference edd quoted is not for "extra s&h and sg&a", but rather a cushion against currency fluctuation. Apple does not want to start suddenly losing money on their products in the UK if the dollar suddenly gets stronger.
Of course Apple could change the price based on exchange rates, but price hikes anger consumers, and there may be laws that make it difficult to suddenly raise the price anyway, especially if they have advertised them in catalogs, etc. And that's not to mention all of the third-parties that would have to sync up with Apple on these changes, and all the wrath they would face from consumers, etc.
So I think that their strategy is to set a price that gives them a currency fluctuation cushion and round it to an attractive number that ends in 9.
474.413 * 1.175 = 557.435 (for current vat)
So the actual current difference is £91.56 They will put that down to shipping & handling costs, administrative costs of operating in the uk.
The iPad has a difference of £52 for a similar priced item (once you take into account vat).