The disability discrimination provisions of the Equality Act 2010, which prohibit unlawful discrimination against a 'disabled person' in employment, define disability as "a physical or mental impairment" which "has a substantial and long-term adverse effect on an individual's ability to carry out normal day-to-day activities".
To fall within this definition, your disability does not need a diagnosis. What matters is the effect of your impairment on your ability to do normal day-to-day activities.