There is much about this issue:
https://www.bbc.co.uk/news/business-52105526
Basically, they must offer a refund, and given a voucher is worthless if the airline goes bust I would choose the cash. Some stranded pax abroad made the mistake of "cancelling" this means their contract with the carrier has ended and they are on their own, one can understand if you are cash strapped. Of course the airlines would rather hang on to your cash and give a voucher. In the case of NAS it seems the credit card companies are retaining the cash rather than passing it on to the company. BA Avios booking system is being wrongly changed to direct folks to vouchers not refunds.