When does the Department of Pensions notify you about your retirement in the UK?

In the UK, the Department for Work and Pensions will notify you 4 months before your normal retirement age to let you know that you can begin the process of claiming your state pension. You can now do this via online if you are within the 4 month window before your state pension age. (See the related link)

Additional information about other types of pension.

In the UK, in addition to the state pension, there are also personal pensions and occupational pensions.

A personal pension is a pension that you set up [ordinarily] yourself, and it is held with a life insurance company or bank.