The British government has said firmly that it will not backtrack on Brexit. But it's unclear whether Article 50 is legally reversible. Former British ambassador to the EU John Kerr, who wrote Article 50, says "it is not irrevocable. You can change your mind while the process is going on."
However, domestic political pressures make it unlikely that the British government would try a U-turn. May will probably take her cue from a catchphrase of predecessor Margaret Thatcher: "The lady's not for turning."