The NHS ( The National Health Service) is the national healthcare system in England. This link sends you to a self-assessment test of your mental health. You will answer a few questions related to mental health. Once all questions have been answered you will be given a depression and anxiety score. 0 is the lowest or happiest and 21 is the highest or saddest. I personally use this every now and again to check up on myself.
Heres my own score from a while back. I recommend doing this test once a week to track your mental health.