Apple isn't overcharging UK customers. It's true that Apple's prices ARE higher in the UK even when you subtract the additional VAT amount that gets added on to the item's US price. However, the REASON for that isn't because Apple is making more profit off of UK customers, rather it's simply because the cost of doing business in the UK is higher than in the US. Remember, the VAT gets added onto EVERYTHING that gets bought in the UK, so Apple has to pay more in the UK just to run its business as usual. For instance, the cost of buying the materials for promotional advertisements like billboards and paper for printing flyers will all have the additional VAT charges added to the price that Apple has to spend. All the Apple store display shelves, store light fixtures, store floor boards, etc. will all have cost Apple higher in the UK due to them having to pay the additional VAT tax on all those products. Etc. etc.
TL;DR Apple isn't overcharging UK customers. Apple's prices in the UK just reflect the additional cost that Apple has to pay in VAT taxes merely to pay for all their normal business supplies.
P.S. I'm not very familiar with how the VAT tax works, so if what I wrote is untrue for some reason, please explain to me specifically what I haven't accounted for.