Source: Data from comScore. Chart presented by comScore at the UKOM & comScore presentation in London, 7th February 2013
45m access via PC, 29m access via mobile, the combined internet reach is 47m, so 2 million (actually 1.7m) only access via mobile (excuding wifi, apps and tablets)
Note - comScore have given me permission to re-publish this, and say:
"The more precise figure is 1.7 million users in the UK only access the internet via mobile.
The number will potentially be higher once we have wi-fi, apps and tablet included in the data."