Main reason tbh is not understanding the effect BREXIT had before it had even been voted on, had always assumed the people in charge of the world economics weren't chimps pulled out of a zoo and could get a decent handle on things. Also forgetting about US state taxes, and how VAT isn't added onto the price post-checkout due to some states not having VAT.
What makes you think dollar prices are too much? They earn 30% more than us on average, so assuming they get hit by the same 20% VAT in whichever state they purchase from, that would be a 10% increase in their prices right? You know that if we earned 30% more than them, I bet my left nut we'd pay 30% more for goods and then juicy VAT on top.
Everything you've written is based on some wrong assumptions. The biggest one is that the
Law of one price holds true, which it does not.
If it were true, you would have already moved to the USA based on the cheaper GPUs for sale there.
In fact, you live in the UK where transportation, taxes, duties, and the 'price that may be realized in an individual market' (to paraphrase wikipedia, some call it gouging, others supply and demand) all result in a different price here to the one in the USA.
As others have noted, if you take any of those $ prices and look at the total cost of importing, it isn't worth it, meaning the price is actually pretty perfectly set. For example, the first one in the OP
ASUS Strix 1070 - $449 / £469
Add some shipping ($20 for argument), sales tax (10% for argument), import tax (20%), and the 1.31 exchange rate and you get... £472.58.
That means there is no arbitrage opportunity (for you anyway, buying retail) to start importing GPUs from the USA and selling them here. That means the price is set properly.
(Edit: there'd be no sales tax actually but there would be some import duty, point still stands)