Hi,
I used to do just what you are aiming for, good luck it is great fun!
You might get lucky with slimming clubs, but the biggest have their own and are quite particular about how they are run. RC is the weirdest and I never ever got a class with them despite having specialist credentials (degrees and exercise certification) and insurances for extremely obese exercisers.
You may also hit lucky with individuals wanting an exercise/life coach. So keep an eye out for more unusual workshops/certificates in nutrition. I also got certified in aquarobics so I could get regular classes in gyms and then Boxercise cos it is a great fun way to exercise and you don't need any equipment to enjoy it (that is also why my signature boxes).
My best advice is to get out there and get seen.... build up 1 or 2 classes at first and then spread slowly. Try getting on local radio or in local newspapers, they always want local good news stories.
Be shameless in your self promotion
Feel free to PM me if you want to chat in private!