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 be helpful to have with you a letter from your hospital doctor. The letter should describe your condition, giving details of any medicines you are taking, and giving information on how to deal with any potential medical problems.
It is helpful to wear a medical alert badge or pendant (from your jewellers or chemist) with emergency contact numbers, especially if you are travelling on your own.