I’m guessing that you know this Mike but for the benefit of new users, we currently use a conversion rate, for changing GBP to USD & vice versa, of the spot rate + 0.45%. This is transparently displayed on the pricing page on our website.
The prices that are displayed in the app when you go to place an order are displayed in both GBP & the converted amount, with that conversion rate applied.
There’s more on how this works in this blog post, under the header “What about US stocks? 
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