23% of UK online purchases are made on mobile devices

"The percentage of online sales completed through a mobile device (smartphones and tablets) has doubled in the space of a year, according to figures from IMRG and Capgemini.
In Q2 of 2012 the percentage stood at 11.6 per cent, but by Q2 of 2013 it had exactly doubled to 23.2 per cent. Over the same period the percentage of online retail site visits through mobile devices has also shot up sharply, from 21.1 per cent in Q2 2012 to 34 per cent in Q3 2013.
IMRG and Capgemini have been tracking sales through mobile devices for over three years, during which time its penetration of online sales has grown by more than 2,000 per cent, with the penetration of online retail visits via mobile devices growing by 1,100 per cent."
Source:  Retail Gazette,3rd September 2013

Note - It doesn't explicitly say that they are UK sales, but I'm assuming that they are