I think you should make it clear in multiple languages (German, English, French, Spanish) that you do accept Euros, but prices are based on GBP, and you take Euros on your OWN exchange rate, not HostelWorld's. As long as it's clear in your HW/HB/Website that you do not charge Euros by the price HW are quoting, you will be fine.
Do you send any confirmation e-mails when they book? If you do, it's worth mentioning the above so they have no comeback. Also, display a notice in reception saying "as according to our booking T&Cs prices are based on GBP, but Euros are accepted at our exchange rate, now HW" or similar. You just want to make sure you have all bases covered.
Most people will be understanding that they have an option to pay in EUR, but have to pay extra.
If it's still a problem, don't accept Euros. Britain is not in the Euro, and you are under no obligation to accept Euros, so bare that in mind.