There is theoretically a risk of damage to the ICD/lead from an electric shock from UK mains buts it’s extremely unlikely and it would be classed as a very low risk.
This is because the current tends not to pass through the ICD/lead but passes through the body to the ground using the shortest and quickest route e.g. hand, up the arm, down the torso, and out through the leg. The majority of devices also have circuitry built in to protect them against electrical surges.
The advice would be to go to the hospital if a person was feeling unwell, otherwise, it would be recommended that a person contact their local device clinic and request a device check which could be performed as a remote transmission. This would pick up on any rare issues caused and provide reassurance.