If you are a UK citizen travelling in the UK and you become ill, you can be treated free of charge by the nearest NHS hospital.
It can help to have a letter from your hospital doctor with you. The letter should describe your condition, giving details of any medicines you are taking and information on how to deal with any potential medical problems.
It helps to wear a medical alert badge or pendant (from a jewellers or chemist) with emergency contact numbers, especially if you are travelling on your own.