It is usually called exercise on prescription, and you need to fit certain criteria - you usually have to attend appointments with a dietician and be above a certain BMI before they will qualify this.
The schemes vary between areas. In my area you get a reduced rate, £2 per session, and it is available to people with a high BMI, asthma, and various other conditions. There is no access to a dietician although they do offer healthy eating group sessions. Ask at your GP surgery what may be available to you.
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