This looks totally acceptable on the assumption you've found good flights from London to Crete and a good ferry from Crete to Naxos. From Naxos I would definitely recommend flying to Athens because you can walk directly from the terminal into a rental car and you'll be in Nafplio in two hours. A little more if you stop to check out Corinth Canal, Acrocorinth, Ancient Corinth or Mycenae, which are all right on the way.
Nafplio is absolutely worth the travel time. I think it's the best single place you can go in Greece to "get it all", ancient, Roman, Byzantine, Venetian. And great examples of those epochs, too, in Mycenae, Epidarus, Argos, Nafplio itself. People ask me a lot to name one place to go in Greece if you're not going to Athens and I say Nafplio.
Drive back to Athens, leave the car in a 10 Euro / Night outdoor lot and then drive it to the airport on last day, drop and walk right into the terminal.