Their system seems determined not to help. It definitely doesn't let you change the currency of your voucher:
https://help.eurostar.com/faq/uk-en/question/Can-I-change-the-currency-of-my-compensation-e-voucher
Unless there's a way to switch currencies at the payment stage of normal online bookings (which I can't check without logging into the Eurostar site), the only way I can find to use the voucher is to book on Eurostar.fr, instead of on Eurostar.com.
You then appear to be offered roughly the same fares (with a £55 fare becoming €63, which is about £56) but you'd need to consider any currency exchange fees your bank might make for paying in euros instead of sterling.
Perhaps their customer helpline can suggest something?
0343 218 6186 (0800-1900 weekdays, 0900-1700 weekends. Calls charged at the same rate as 01 or 02 numbers)