When travelling with a pre-existing medical condition most holiday company insurance policies will not cover these conditions. However there are specialist insurance companies that will do so.
Before applying for travel insurance you will need to contact your doctor for written permission because the majority of travel companies will not provide coverage if you are travelling against your doctor’s orders. The insurance company will also usually ask you to go through a medical screening process. It is vital that you always disclose any pre-existing medical conditions and failure to do so can lead to subsequent claims becoming invalidated.
Ever since the 1st January 2006 the European Health Insurance Card (EHIC) can be used by citizens of European Economic Area (EEA) countries and Switzerland to cover any necessary medical treatment due to either accident or illness. Treatment covered by an EHIC is that which is seen to be clinically necessary in the opinion of a doctor or dentist. The card also covers pre-existing conditions including routine monitoring of conditions such as diabetes. Although also covered by the card, anyone requiring dialysis or oxygen therapy must make arrangements prior to departure from their home member state.
More information about applying for an EHIC is available from the NHS. And there is a leaflet for visitors to the UK at this link.