If you want Avios you are pretty limited with options.

HSBC Premier Mastercards give you an option via linking with Curve and earning Avios, but you obviously need to tick the requirements for a premier account.

However seeing as the majority of credit cards have an FX fee, unless your employer is willing to give you a corporate card I can't see they have any argument. They can't force you to use a debit card, and can't force you to apply for another credit card. A corporate card would probably be the worst solution for you as generally you get no rewards.

Curve really seems the best option, but maybe worth looking at other earning credit cards - Virgin, IHG etc?