Speech therapy is available free under the NHS and in most areas you can refer yourself, without having to see your GP first. You may be offered individual therapy, group therapy or both. Unfortunately, due to funding constraints, the service isn't available in all areas.
BSA can give you local contact details so that you can enquire about what is available in your area: contact our Information and Support Service. If possible, it is best to see a therapist who specialises in stammering. If your local NHS Trust doesn't have a service for adults, or doesn't have a stammering specialist, it is worth asking if you can be referred to a neighbouring Trust or to City Lit.
There is more at Finding a qualified speech and language therapist, including the option of a private therapist.