The currency in Greece is the Euro. ATMs accepting all major cards will be found in all resort towns and many hotels will be able to change foreign currencies.
The official language is Greek though English is unsurprisingly widely spoken in the more touristy parts of the island. This said, if you break down in a remote rural part of the island, don't expect to be able to make yourself understood to locals!
The code for Greece is +30 and for Zante is 2695.
Visas are not required for entry to Greece by UK citizens.