My back-of-the-envelope calculation suggests that the break-even point is around £18,600: you win by about 2p on every pound you earn above that, and lose below it.

There are benefits changes at the bottom end which presumably ameliorate the doubling of 10% rate to some extent (e.g. working tax credit) but I'm afraid I don't know how those benefits work, so if you're affected by those you'll have to work it out yourself.