Your cards will work, but retailers and restaurant owners may insist on cash.
If you're looking for a refund, do be aware: the only time you can expect to easily get one from the tour operator is when the Foreign Office advises against travel. This has not happened with Greece.
As for protection, package holidays are covered by the Civil Aviation Authority's ATOL (Air Travel Organisers' Licensing) scheme, which means you are protected from being stranded abroad.
You won't have this protection if you have booked direct with an airline or accommodation provider unless either of those goes bust, in which case if you have paid more than £100 by credit card you will be covered for the service you have lost. Check your travel insurance.
Some policies may not cover insolvency. Those that do are likely to cover only the service you have lost.
