If you have an urgent health problem when the practice is closed you should dial 111. This is a 24-hour a day, free to call (even from mobile phones) NHS advice line.
The trained health advisors at NHS111 will take your details and ask a series of questions designed to identify the type of service that is best able to help you and a safe timescale for this happen within. Whilst some of the questions can seem odd the system is set up to rapidly pick up people with very serious or life-threatening problems so that they can get emergency help, before moving through the other possibilities. On some calls you may also speak to a nurse or paramedic who will provide further assessment when needed. The whole approach is approved by several of the medical Royal Colleges and has taken millions of calls effectively and safely.
At the end of the call you may be referred to a specific service, such as the local out-of-hours GP, or be given some self-care advice that should enable you to look after the problem safely at home.