Now that Ofcom has allowed the networks to re-farm their existing 2G spectrum for 3G use, this will make the transition a lot smoother than it would have done if 3G was limited to 2100MHz only.
O2 has already started upgrading its 900MHz spectrum to run both 2G and 3G services. I imagine that a phased approach will eventually see the balance switch from 2G to 3G and 2G eventually phased out. Apart from a few Blackberry handsets there can't be that many 2G only handsets around these days surely!