If you are ill while away from home or if you are not signed up with with a doctor but need to see one you can receive emergency treatment from the local GP practice for 14 days. After 14 days you will need to sign up with that practice as a temporary or permanent patient.
You can be a temporary patient for up to three months. This will allow you to be on the local practice list and still remain a patient of your permanent GP. After three months you will have to re-register as a temporary patient or permanently sign up with that practice.
To sign up as a temporary patient simply contact the local practice you wish to use. Practices do not have to accept you as a temporary patient although they do have an obligation to offer emergency treatment. You cannot sign up as a temporary patient at a practice in the town or area where you are already registered with a surgery.
There is information about this on the NHS registration page.