It's cheaper! No it's not! Yes it is!
In reality it's not that difficult to work out. Let's look at the same unit in the UK and say the US.
2.4Ghz 13" base model:
Retail for that is GBP999 right? That includes sales tax (VAT) and all import duties. To compare to the US you'd need to remove the local sales tax.
Now let's look at the same model in the US:
Let's pretend we're buying that in downtown New York so you need to add in state/city sales tax which if memory serves is 8.875%. So, total price to you at the Apple Store is US1305.41.
This is about GBP819.78 depending on what day/exchange rate you use.
Ok - cheaper you think! Firstly, consider that the rate in conversion is a bank conversion rate - the 'real' rate you'd pay would likely be slightly higher.
Here's the thing though - to import legally you have to cover off import duty & VAT. Apply the duty/VAT you get this:
So you'll pay GBP963.42 to get the same laptop. Not including shipping or transport of course.
You'll save 36 quid.
Your other option is to try and evade VAT of course, saving you some more - but then you're not really comparing prices as you're breaking the law
I also *think* you may be able to reclaim some of the sales tax when you leave the US? That shows understanding of sales tax though and if you then land and try and evade UK VAT then they're likely to go to town on you on evasion. If you pay US sales tax, don't bother reclaiming and try and evade you may be let through on the assumption of ignorance (you'd have to pay VAT of course).
It is slightly cheaper to buy a lot of electronic gear in the US - but the gap is nothing like the direct $ to GBP price comparison - there's different structures in pricing and duties in the US for start.
Do a correct, legal, comparison and you'll find that that gap isn't that big.
It only really comes into it's own if you want to evade the UK taxes.