You can only be registered to one GP in the UK and it has to be near your residential address. Once you have registered, you can only go to that clinic. You may change clinics but first, you will need to unregister from the first one. Most clinics are open 8am-6pm, Mon-Fri, and most of them don’t accept appointments in advance which means that you’ll have to call in first thing in the morning and check their availability.