To count as a disability, under the terms of the 2010 Equality act, a condition must substantially affect your ability to carry out day to day tasks.
If you're asking for the purposes of DSAs, you will need to ask your doctor/nurse practitioner or similar health professional to provide written confirmation of your diagnosis and a description of how the condition(s) affect your ability to carry out day to day tasks, or a copy of your post age 16 psychologist's report if your're claiming for a learning difficulty such as Dyslexia. If you're accepted as eligible for DSAs, you'll be invited to book a study needs assessment and it is in this that the effects your condition has on your studies will be looked at.