As always, US pries are ex-tax (meaning state sales taxes are added on and can be as much as 10% depending on city and state). Most other places (UK included) as inc-tax. If you take out the 20% VAT you get to 349 GBP, which is ~437 USD at current exchange rates. That's not that far off.
Interesting, I'd find that super annoying as a consumer that's used to seeing the full price (ex. delivery) upfront. Is this standard practice for online US stores?
It is annoying but taxes in the US are based on locality, so they involve state and municipal sales taxes, so it would be impossible to list one price for every customer.