The reason we pay higher in the UK is because the price is determined by whatever they think they can get away charging in a particular country, rather than what they should reasonably be selling it for.
I remember years ago I used to renew my Norton AV subscription from the US site as it was considerably cheaper than the UK one.
I can't remember the exact prices but I think it was about Â£15 here and $13 on the US website and that's at the time when there were nearly two Dollars to the Pound.
Then Symantec blocked this so that UK users had to renew subscriptions from the UK site.
TBH I can't really think of a better example of blatant profiteering as the cost of providing a renewed serial number online is the same regardless of where the customer is based.