The payment processor (either Paypal or Stripe) will convert the sale to the dollar amount at the checkout so you will pay in USD (this varies slightly depending on the day’s exchange rate)
You can get an idea of today’s rate here: XE currency convertor US Dollars
As a guide, £49 GBP is around $60.45 (this might vary slightly depending on the exchange rate the payment processor uses)
And do you accept American Express?
Yes, the Stripe payment processor accepts American Express
p.s Here’s a Euro exchange rate as well. £49 GBP is around 56 EUR XE currency convertor Euro