You can't set the price in anything other than USD, as far as I know.
When I started selling here, I took what I charged in GBP, converted it, and rounded it to an even number of USD. (For example, putting $40 instead of $39.86) It might be 25p, 50p different to what I'd charge if selling in the UK, but I doubt anyone is going to fuss that much about such a small amount.
I've found it's best not to get too nit-picky over getting prices to match exactly, as the conversion rate fluctuates constantly (and can go quite wild at times, as it did for a while after Brexit) you'll only drive yourself mad. Find a number that more-or-less matches your GBP price, but that also looks nice in USD. And pray that the conversion stays relatively stable for a while, so you don't have to adjust it every month, haha... Best of luck with sales!