From Mind website:

The 1 in 4 statistic

1 in 4 people will experience a mental health problem in any given year.

This is the most commonly quoted statistic, and the one which has the most research evidence to support it. It came initially from a large scale study published first in 1980, then updated again 1992. This figure is further supported by the results of all three Adult Psychiatric Morbidity Surveys

The breakdown below gives an overview of what treatment those who experience mental health problems are likely to seek and get:

• around 300 people out of 1,000 will experience mental health problems every year in Britain
• 230 of these will visit a GP
• 102 of these will be diagnosed as having a mental health problem
• 24 of these will be referred to a specialist psychiatric service
• 6 will become inpatients in psychiatric hospitals.