Used this Amex voucher site before to get House of Fraser vouchers and had a look today and it appears to be showing 50% off HMV vouchers. You have to login to view the offer. You'll need an Amex card to complete purchase.
It does state that it is for American Express Employees but not sure what checks they do. You maybe able to get away with it. Worst case scenario they refuse you and you get a refund.
Can be worthwhile if you want to buy a console from in store. As these are paper vouchers, they have to be spent in store although I don't think you can buy online from HMV anymore anyway.