Prices for most things are advertised with VAT in the UK since virtually everyone has to pay it and those who don't (businesses) are generally not going to complain when their purchases come in lower than expected.
Its not illegal to advertise without VAT (Computers are traditionally an area where this happens a lot), its just that it really pisses people off when they think they are paying so much and then the price gets hiked another 20% at the checkout. Its their fault, but that doesn't matter to most consumers. They are still gonna get pissed off.
Its fine to argue that Apple shouldn't have to absorb the costs of other countries tax increases, or rather it would be if they didn't already factor in massive price buffers against changes in the exchange rates. Its stupid for anyone who keeps track of Apple to expect them to absorb costs when they have an apparent excuse not to.
Take the Apple TV. $99 in the US right now which equates to £63.31 at current exchange rate. Add the new 20% VAT and you get to £75.97. It retailed yesterday at £99 Which means Apple have added a whopping 33% "buffer profit" on top in case the dollar suddenly tanks against sterling. And they still saw fit to bump the price of this product by the extra £2 for the VAT increase. THIS is why savvy UK consumers get pissed off with Apple. We expect to pay higher prices than we would for PCs, just like you do in the states. Its fine. But to add an extra third just in case the exchange rates change? Whats wrong with just waiting for them to change and adjusting the prices IF they do? How about you review the prices every month or two? I personally would prefer that to having to pay an extra 10-30% premium on top of our already ridiculous, overtaxed prices.
Lets also bear in mind that most Apple products are making margins in the region of 50% to begin with, making their exchange rate buffer even less forgivable.
The exchange buffer drops as you spend more, but its still over 10% on a MacBook, and around 7% on a stock Mac Pro.