In an emergency, call 999 (this is the number for Fire and Police emergencies as well).
All hospitals have an Emergency Department and doctors surgeries are open normally 9 am - noon and also for several hours in the afternoon/evening.
EU citizens normally receive treatment without charge, other nationalities will need some kind of health insurance. Check with your local health authority before you leave, or call the National Health Service on +44 0845 4647 for more information.