Multicurrency coupons

We sell coupons successfully and have them disabled for international users. Eg US customers can buy and use $50 USD coupons, UK customers can’t see or buy these.
How would we go about creating a 50 GBP voucher which is available to UK customers, costs them 50 GBP, and is redeemable with a value of 50 GBP?
It seems that when a voucher is purchased, its value is stored in the shop’s default currency (USD), so the vouchers value is determined by the exchange rate at time of use.