In a recent press release, 3UK says their 3G network now transports 100 terabytes of data a day. An impressive number. One and a half years ago, 3UK reported 1000 TB per month or roughly 33 TB per day. In other words, the amount of data going through their network has trippled in about 18 months.
Back then I did some calculations to set the number into context and to show by breaking it down to a base station, there's still ample room for future growth. At the time I estimated them to have around 8000 base stations. The press report linked to above states that after their recent network upgrade and extension they now have increase their number of base station sites from 7500 to 12400 in the last 3 years. So my estimation back then was right on the mark.
With the number of base stations having increased by a third, the load of their network as calculated back in 2009 should by now have doubled. So from a capacity point of view they should still be fine and HSPA+ in addition to adding a second carrier per coverage sector should keep them ahead of the increasing use in terms of capacity for quite some time if they do things right.