There are two sources of information in UK.

Office for national statistics are nice. They're robust about what does or doesn't count, and how they count, and where you need to be cautious with the numbers. http://www.ons.gov.uk/

The other place might be Public Health Outcomes: http://www.phoutcomes.info/

EG http://fingertips.phe.org.uk/profile-group/mental-health

And finally NICE tend to have excellent referencing.

Noodle around here to find the NICE guidance. http://pathways.nice.org.uk/pathways/drug-misuse