It was an error to leave but this does not mean that it would be a good idea to go back in. What would make sense is further alignment with the bloc; economically beneficial all around and would not cause political difficulties, at least not for a Labour government. The present state of affairs benefits precisely no one.
So you're saying the UK's best course of action is to negotiate similar (though perhaps not identical) terms that Switzerland currently enjoys with the European Union?
The Iceland/Norway arrangements are saner than Switzerland’s mess of treaties.