I disagree actually... not that th 50mm isn't sharp etc, but that you're really talking about upgrading your kit lens.
The most direct, higher quality replacement is the
Sigma 18-50mm f/2.8-4.5 DC OS which is £190. A step above that is the
Sigma 17-70mm f/2.8-4.5 which is excellent value at £250 (ish).
Your other option would be to extend the range a little bith with either the
Canon 18-135mm IS which is £300 or the
Sigma 18-125mm OS which is £250.
The Sigma 18-125mm is basically the same as the Nikkor 18-105mm that I've got and a recent back to back test of them showed no optical differences. The only real difference was that the Sigma is slightly better built. I mention this because the 18-105mm I have is far superior to an 18-55mm kit lens, and either the Canon or Sigma here would be a huge step forward.