In the UK, having an ICD or the underlying heart condition that led to its implantation will almost always prevent you from holding a Civil Aviation Authority (CAA) medical certificate, which is required to fly as a pilot of any category.
This applies to Private Pilot Licences (PPL), Commercial Pilot Licences (CPL), and Airline Transport Pilot Licences (ATPL). The CAA assesses each case individually, but conditions such as ventricular arrhythmia and ICD implantation are considered disqualifying in the vast majority of circumstances due to the safety implications of sudden incapacitation at altitude.
If you are a trained pilot and are facing ICD implantation, it is worth seeking specialist aviation medical advice early — ideally from an Aeromedical Examiner (AME) — before any decisions are made, so that all options are fully understood. In very limited circumstances, some pilots may be able to fly in certain categories with specific conditions, but this is rare and assessed on a case-by-case basis.
More information is available from the UK Civil Aviation Authority at www.caa.co.uk.