Unfortunately it seems a lot of people are having this problem at the moment. Consumer group Which? are really pushing on this now, because there’s up to £7bn owed for cancelled trips.
Airlines are hesitant to offer cash refunds because they fear it will push them into bankruptcy, but obviously this is destroying customer faith. Which? are asking the government help intervene to protect travel companies, and that any vouchers/credit notes should be guaranteed against insolvency.
It’s an ongoing issue, with no simple answer, so it’s probably worth making sure the airline is aware that you’re not satisfied with a credit note (although it will probably be hard to contact them) and keeping an eye on what happens as more pressure is put on them to do more.
We’ll certainly be keeping it under-review. Good luck!