If you need to telephone for an ambulance you should dial 999 (the call is free). You will be asked to explain the medical emergency. An emergency response vehicle will be sent to your location to take you/the patient to an accident and emergency department (A&E). You can also visit the A&E of your local hospital.
The emergency service is free to everyone in the first instance, regardless of your status in the UK. However, following emergency treatment if you are admitted to hospital and you are liable to charging for NHS services, you will have to pay for any subsequent treatment.