The 10GB estimate was that, seems this research gives me a new figure of 14.9GB.
Taking some figures from the 2010 OFCom report:
p226:...Western Europe (7.4PB per million)...
Assuming the UK is typical of Western Europe, has a population around 70 million and there are 20 million broadband lines (link), crunching the numbers around and adding some for mobile internet, the average comes out at around 3GB/month.
I don't think I've got the numbers wrong, but it's a heck of a discrepancy... 
edit- add link for BB lines.
You will distort the average speed figure by including mobile so I suspect a reasanably true average speed is about 5Gb which is pretty slow by european figures.
The data also shows that Broadband takeup in the UK is very high at 20 Million out of a population of about 66 Million.
It is perhaps more meaningful to base it on Household rather than population. I think the average household size is about 2.5 so take up is in the region of 70% plus
Another factor with regard to average speeds is that at present there is a two class system in the UK. In the Cabled areas speeds of upto 50MB are available and the average speed in Cabled homes is probably over 15Gb. The small number of Invenity enabled areas will also have highers sppeds typically up to 30GB
If you take the average speeds in areas without cable & Invinity you tend to get a very different picture with an average of below 5Gb . So the UK is not actually doing very well at all